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

timesheets.example.toml

@@ -0,0 +1,5 @@
+[joplin]
+token = "your_api_token_here"
+
+[projects]
+map = "/path/to/project_map.json"