fix(sleep): add Windows AppData support for default OpenCode database discovery (#56)

[RohithPariki]

Summary

Fixes #56 by adding native Windows %LOCALAPPDATA% and %APPDATA% environment variable resolution to default_opencode_db().

When running on Windows systems with XDG_DATA_HOME unset, skillopt-sleep --source opencode previously defaulted to ~/.local/share/opencode/opencode.db (a POSIX path that does not exist on native Windows). OpenCode on Windows stores its database under %LOCALAPPDATA%\opencode\opencode.db or %APPDATA%\opencode\opencode.db.

---

Key Changes

1. skillopt_sleep/harvest_opencode.py:

   • Updated default_opencode_db() to inspect sys.platform == "win32" and check LOCALAPPDATA and APPDATA environment variables before falling back to ~/.local/share.

2. tests/test_harvest_opencode.py:

   • Added test_default_database_honors_windows_appdata to verify Windows environment variable path resolution for default_opencode_db().

3. Documentation (docs/reference/cli.md & plugins/README.md):

   • Documented the Windows %LOCALAPPDATA% / %APPDATA% default database path auto-discovery.

---

Verification

• Ran unit tests: pytest tests/test_harvest_opencode.py (28/28 passed).
• Ran code style check: ruff check skillopt_sleep/harvest_opencode.py tests/test_harvest_opencode.py (All checks passed!).