feat: implement core auction logic and env loader

- Add auction.py with add_bid and find_winner functions
- Add env.py with stdlib-based .env loader (no python-dotenv)
- Add tests/test_auction.py with test cases for auction functions
- Remove .env.example (tokens not meant to be committed)
- Update docs/plan.md to mark Milestone 2 as complete

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/env.py

@@ -0,0 +1,38 @@
+"""! @file env.py
+@brief Minimal `.env` file loader using only the standard library.
+"""
+
+import os
+from pathlib import Path
+
+
+def parse_env(text: str) -> dict[str, str]:
+    """! @brief Parse `KEY=VALUE` lines.
+    @param text Contents of a `.env` file. Blank lines and `#` comments are ignored.
+    @return Dictionary of parsed variables; surrounding quotes are stripped.
+    """
+    values: dict[str, str] = {}
+    for line in text.splitlines():
+        line = line.strip()
+        if not line or line.startswith("#") or "=" not in line:
+            continue
+        key, value = line.split("=", 1)
+        value = value.strip()
+        if len(value) >= 2 and value[0] == value[-1] and value[0] in "\"'":
+            value = value[1:-1]
+        values[key.strip()] = value
+    return values
+
+
+def load_env(path: str | Path) -> dict[str, str]:
+    """! @brief Load a `.env` file into `os.environ` without overriding existing variables.
+    @param path Path to the `.env` file; a missing file is not an error.
+    @return Dictionary of the variables read from the file.
+    """
+    file = Path(path)
+    if not file.is_file():
+        return {}
+    values = parse_env(file.read_text(encoding="utf-8"))
+    for key, value in values.items():
+        os.environ.setdefault(key, value)
+    return values