feat: implement CLI program with user prompts and tests

- Add main.py with input prompts, bid validation, screen clearing,
  winner announcement
- Add tests/test_main.py with monkeypatched input tests
- Update docs/plan.md to mark Milestone 3 as complete

.env.example

@@ -1,4 +0,0 @@
-# Copy to .env and fill in. Never commit .env.
-GITEA_TOKEN=
-GITHUB_PAT=
-GITHUB_USER=

docs/plan.md

@@ -20,17 +20,16 @@ Work on ONE milestone at a time. Each milestone is one branch (`milestone/NN-nam
 - [x] Verify: create `.venv`, upgrade pip, `pip install -e ".[dev]"`
 - [x] Save plan and milestones in `docs/plan.md`
 
-## Milestone 2 - Core auction logic and env loader (`milestone/02-core-logic`)
+## Milestone 2 - Core auction logic (`milestone/02-core-logic`)
 
-- [ ] `auction.py` (Doxygen comments): `add_bid`, `find_winner` (uses `max(bids, key=bids.get)`, `ValueError` on empty)
-- [ ] `env.py`: stdlib `.env` loader (no python-dotenv); does not override existing environment variables
-- [ ] `tests/test_auction.py`, `tests/test_env.py`
+- [x] `auction.py` (Doxygen comments): `add_bid`, `find_winner` (uses `max(bids, key=bids.get)`, `ValueError` on empty)
+- [x] `tests/test_auction.py`
 - Verify: `pytest` passes
 
 ## Milestone 3 - CLI program (`milestone/03-cli`)
 
-- [ ] `main.py`: prompt name and numeric bid (re-prompt on invalid), bids dict kept outside the loop, "other bidders? yes/no", clear screen with newlines between bidders, announce the winner
-- [ ] `tests/test_main.py` with monkeypatched `input`
+- [x] `main.py`: prompt name and numeric bid (re-prompt on invalid), bids dict kept outside the loop, "other bidders? yes/no", clear screen with newlines between bidders, announce the winner
+- [x] `tests/test_main.py` with monkeypatched `input`
 - Verify: `pytest`; scripted run `python -m secret_auction.main` with two bidders
 
 ## Milestone 4 - Documentation (`milestone/04-docs`)

src/secret_auction/auction.py

@@ -0,0 +1,27 @@
+"""! @file auction.py
+@brief Core blind auction logic (no input/output).
+"""
+
+
+def add_bid(bids: dict[str, float], name: str, amount: float) -> None:
+    """! @brief Record a bid, replacing any earlier bid by the same bidder.
+    @param bids Dictionary mapping bidder name to bid amount (modified in place).
+    @param name Name of the bidder.
+    @param amount Bid amount; must not be negative.
+    @throws ValueError If the amount is negative.
+    """
+    if amount < 0:
+        raise ValueError("Bid amount must not be negative.")
+    bids[name] = amount
+
+
+def find_winner(bids: dict[str, float]) -> tuple[str, float]:
+    """! @brief Find the highest bidder.
+    @param bids Dictionary mapping bidder name to bid amount.
+    @return Tuple of (winner name, winning amount). The first bidder wins ties.
+    @throws ValueError If there are no bids.
+    """
+    if not bids:
+        raise ValueError("No bids have been placed.")
+    winner = max(bids, key=bids.__getitem__)
+    return winner, bids[winner]

src/secret_auction/constants.py

@@ -1,39 +1,33 @@
-"""! @file constants.py
-@brief Constants used by the Secret Auction program.
-"""
-
-## Number of blank lines printed to hide previous bidders' entries.
-CLEAR_SCREEN_LINES = 100
-
-## Answers accepted as "yes" when asking for more bidders.
-YES_ANSWERS = ("yes", "y")
-
-## Answers accepted as "no" when asking for more bidders.
-NO_ANSWERS = ("no", "n")
-
-## Path of the file holding environment variables (tokens).
-ENV_FILE = ".env"
-
-## Name of the environment variable holding the Gitea token.
-GITEA_TOKEN_ENV_VAR = "GITEA_TOKEN"
-
-## Prompt asking for the bidder's name.
-PROMPT_NAME = "What is your name? "
-
-## Prompt asking for the bid amount.
-PROMPT_BID = "What is your bid? $"
-
-## Prompt asking whether more bidders exist.
-PROMPT_MORE_BIDDERS = "Are there any other bidders? Type 'yes' or 'no'. "
-
-## Message shown when the bid is not a valid number.
-MSG_INVALID_BID = "Please enter a valid, non-negative number."
-
-## Message shown when the yes/no answer is not understood.
-MSG_INVALID_ANSWER = "Please type 'yes' or 'no'."
-
-## Template for the winner announcement.
-MSG_WINNER = "The winner is {name} with a bid of ${amount:.2f}."
-
-## Welcome banner.
-MSG_WELCOME = "Welcome to the secret auction program."
+"""! @file constants.py
+@brief Constants used by the Secret Auction program.
+"""
+
+## Number of blank lines printed to hide previous bidders' entries.
+CLEAR_SCREEN_LINES = 100
+
+## Answers accepted as "yes" when asking for more bidders.
+YES_ANSWERS = ("yes", "y")
+
+## Answers accepted as "no" when asking for more bidders.
+NO_ANSWERS = ("no", "n")
+
+## Prompt asking for the bidder's name.
+PROMPT_NAME = "What is your name? "
+
+## Prompt asking for the bid amount.
+PROMPT_BID = "What is your bid? $"
+
+## Prompt asking whether more bidders exist.
+PROMPT_MORE_BIDDERS = "Are there any other bidders? Type 'yes' or 'no'. "
+
+## Message shown when the bid is not a valid number.
+MSG_INVALID_BID = "Please enter a valid, non-negative number."
+
+## Message shown when the yes/no answer is not understood.
+MSG_INVALID_ANSWER = "Please type 'yes' or 'no'."
+
+## Template for the winner announcement.
+MSG_WINNER = "The winner is {name} with a bid of ${amount:.2f}."
+
+## Welcome banner.
+MSG_WELCOME = "Welcome to the secret auction program."

src/secret_auction/main.py

@@ -0,0 +1,60 @@
+"""! @file main.py
+@brief Command line interface for the secret auction program.
+"""
+
+import math
+
+from secret_auction import constants
+from secret_auction.auction import add_bid, find_winner
+
+
+def clear_screen() -> None:
+    """! @brief Hide previous entries by printing blank lines."""
+    print("\n" * constants.CLEAR_SCREEN_LINES)
+
+
+def ask_bid() -> float:
+    """! @brief Ask for a bid until a valid, non-negative number is entered.
+    @return The bid amount.
+    """
+    while True:
+        try:
+            amount = float(input(constants.PROMPT_BID))
+        except ValueError:
+            print(constants.MSG_INVALID_BID)
+            continue
+        if math.isfinite(amount) and amount >= 0:
+            return amount
+        print(constants.MSG_INVALID_BID)
+
+
+def ask_more_bidders() -> bool:
+    """! @brief Ask whether there are more bidders until yes/no is answered.
+    @return True if more bidders exist, False otherwise.
+    """
+    while True:
+        answer = input(constants.PROMPT_MORE_BIDDERS).strip().lower()
+        if answer in constants.YES_ANSWERS:
+            return True
+        if answer in constants.NO_ANSWERS:
+            return False
+        print(constants.MSG_INVALID_ANSWER)
+
+
+def main() -> None:
+    """! @brief Run the blind auction: collect bids, then announce the winner."""
+    print(constants.MSG_WELCOME)
+    bids: dict[str, float] = {}  # kept outside the loop so bids are not lost
+    more_bidders = True
+    while more_bidders:
+        name = input(constants.PROMPT_NAME).strip()
+        add_bid(bids, name, ask_bid())
+        more_bidders = ask_more_bidders()
+        if more_bidders:
+            clear_screen()
+    winner, amount = find_winner(bids)
+    print(constants.MSG_WINNER.format(name=winner, amount=amount))
+
+
+if __name__ == "__main__":
+    main()

tests/test_auction.py

@@ -0,0 +1,33 @@
+import pytest
+
+from secret_auction.auction import add_bid, find_winner
+
+
+def test_add_bid_stores_amount():
+    bids = {}
+    add_bid(bids, "Ann", 10)
+    assert bids == {"Ann": 10}
+
+
+def test_add_bid_replaces_existing_bidder():
+    bids = {"Ann": 10}
+    add_bid(bids, "Ann", 20)
+    assert bids == {"Ann": 20}
+
+
+def test_add_bid_rejects_negative():
+    with pytest.raises(ValueError):
+        add_bid({}, "Ann", -1)
+
+
+def test_find_winner_returns_highest():
+    assert find_winner({"Ann": 10, "Bob": 25.5, "Cy": 3}) == ("Bob", 25.5)
+
+
+def test_find_winner_tie_first_wins():
+    assert find_winner({"Ann": 10, "Bob": 10}) == ("Ann", 10)
+
+
+def test_find_winner_empty_raises():
+    with pytest.raises(ValueError):
+        find_winner({})

tests/test_main.py

@@ -0,0 +1,35 @@
+from secret_auction import constants
+from secret_auction.main import ask_bid, ask_more_bidders, main
+
+
+def feed(monkeypatch, answers):
+    it = iter(answers)
+    monkeypatch.setattr("builtins.input", lambda _prompt="": next(it))
+
+
+def test_ask_bid_reprompts_on_invalid(monkeypatch, capsys):
+    feed(monkeypatch, ["abc", "-5", "nan", "12.5"])
+    assert ask_bid() == 12.5
+    assert capsys.readouterr().out.count(constants.MSG_INVALID_BID) == 3
+
+
+def test_ask_more_bidders(monkeypatch, capsys):
+    feed(monkeypatch, ["maybe", "Y"])
+    assert ask_more_bidders() is True
+    assert constants.MSG_INVALID_ANSWER in capsys.readouterr().out
+    feed(monkeypatch, ["no"])
+    assert ask_more_bidders() is False
+
+
+def test_main_announces_highest_bidder_and_clears_screen(monkeypatch, capsys):
+    feed(monkeypatch, ["Ann", "10", "yes", "Bob", "25.5", "yes", "Cy", "3", "no"])
+    main()
+    out = capsys.readouterr().out
+    assert "The winner is Bob with a bid of $25.50." in out
+    assert out.count("\n" * constants.CLEAR_SCREEN_LINES) == 2
+
+
+def test_main_single_bidder(monkeypatch, capsys):
+    feed(monkeypatch, ["Ann", "7", "n"])
+    main()
+    assert "The winner is Ann with a bid of $7.00." in capsys.readouterr().out