Add Caesar cipher project with Doxygen docs

src/caesar_cipher/env.py

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+## @file env.py
+#  @brief Minimal stdlib `.env` reader (no third-party dependency).
+
+import os
+from pathlib import Path
+
+from caesar_cipher.constants import ENV_FILE
+
+
+def load_env(path: str | Path = ENV_FILE) -> dict[str, str]:
+    ## @brief Read KEY=VALUE pairs from an env file.
+    #  @param path Path of the env file.
+    #  @return Mapping of variables; empty if the file does not exist.
+    values: dict[str, str] = {}
+    env_path = Path(path)
+    if not env_path.is_file():
+        return values
+    for line in env_path.read_text(encoding="utf-8").splitlines():
+        line = line.strip()
+        if not line or line.startswith("#") or "=" not in line:
+            continue
+        key, _, value = line.partition("=")
+        values[key.strip()] = value.strip().strip("\"'")
+    return values
+
+
+def get_token(name: str, path: str | Path = ENV_FILE) -> str | None:
+    ## @brief Get a token from the process environment or the env file.
+    #  @param name Variable name, e.g. GITEA_TOKEN.
+    #  @return The token or None when not set.
+    return os.environ.get(name) or load_env(path).get(name)