feat(config): add TOML config file support

- Add config.py with load_config(), find_default_config(), get_token(),
  and get_map_path()
- Auto-discover timesheets.toml in cwd; override with --config flag
- Priority: CLI flag > config file > env var / cwd default
- Add timesheets.example.toml as a committed reference template
- Add timesheets.toml to .gitignore to prevent accidental secret leakage
- Document config file format in AGENTS.md

.gitignore

@@ -8,3 +8,6 @@ wheels/
 
 # Virtual environments
 .venv
+
+# Local config (contains secrets)
+timesheets.toml