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

AGENTS.md

@@ -17,6 +17,7 @@ timesheets/
     ├── parser.py     # markdown table parsing, aggregation, date filtering
     ├── projects.py   # project_map.json loading and key resolution
     ├── output.py     # CSV writing and summary printing
+    ├── config.py     # TOML config file loading and key extraction
     ├── joplin.py     # Joplin API integration (notebook traversal, note fetching)
     └── utils.py      # shared low-level helpers (duration parsing, formatting, etc.)
 ```
@@ -28,6 +29,7 @@ tests/
 ├── test_utils.py
 ├── test_parser.py
 ├── test_projects.py
+├── test_config.py
 ├── test_output.py
 └── test_joplin.py
 ```
@@ -95,6 +97,22 @@ The API token can be provided via:
 
 ---
 
+## Config file
+
+Create `timesheets.toml` in your working directory (or pass `--config /path/to/file.toml`):
+
+```toml
+[joplin]
+token = "your_api_token_here"
+
+[projects]
+map = "/path/to/project_map.json"
+```
+
+Priority order for each value: **CLI flag > config file > environment variable / default**.
+
+---
+
 ## Joplin notebook structure
 
 The `--joplin` flag expects the following notebook hierarchy in Joplin: