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

tests/test_config.py

@@ -0,0 +1,88 @@
+import sys
+from pathlib import Path
+
+import pytest
+
+from timesheets.config import (
+    DEFAULT_CONFIG_FILENAME,
+    find_default_config,
+    get_map_path,
+    get_token,
+    load_config,
+)
+
+# ---------------------------------------------------------------------------
+# load_config
+# ---------------------------------------------------------------------------
+
+
+class TestLoadConfig:
+    def test_none_returns_empty(self):
+        assert load_config(None) == {}
+
+    def test_loads_valid_toml(self, tmp_path):
+        f = tmp_path / "timesheets.toml"
+        f.write_text('[joplin]\ntoken = "abc123"\n')
+        result = load_config(f)
+        assert result == {"joplin": {"token": "abc123"}}
+
+    def test_missing_file_exits(self, tmp_path, capsys):
+        with pytest.raises(SystemExit):
+            load_config(tmp_path / "nonexistent.toml")
+        assert "not found" in capsys.readouterr().err
+
+    def test_invalid_toml_exits(self, tmp_path, capsys):
+        f = tmp_path / "bad.toml"
+        f.write_text("not = valid = toml\n")
+        with pytest.raises(SystemExit):
+            load_config(f)
+        assert "could not parse" in capsys.readouterr().err
+
+    def test_accepts_str_path(self, tmp_path):
+        f = tmp_path / "timesheets.toml"
+        f.write_text('[projects]\nmap = "/some/path.json"\n')
+        result = load_config(str(f))
+        assert result["projects"]["map"] == "/some/path.json"
+
+
+# ---------------------------------------------------------------------------
+# find_default_config
+# ---------------------------------------------------------------------------
+
+
+class TestFindDefaultConfig:
+    def test_returns_none_when_absent(self, tmp_path, monkeypatch):
+        monkeypatch.chdir(tmp_path)
+        assert find_default_config() is None
+
+    def test_returns_path_when_present(self, tmp_path, monkeypatch):
+        monkeypatch.chdir(tmp_path)
+        cfg = tmp_path / DEFAULT_CONFIG_FILENAME
+        cfg.write_text("")
+        result = find_default_config()
+        assert result == cfg
+
+
+# ---------------------------------------------------------------------------
+# get_token / get_map_path
+# ---------------------------------------------------------------------------
+
+
+class TestGetters:
+    def test_get_token_present(self):
+        assert get_token({"joplin": {"token": "xyz"}}) == "xyz"
+
+    def test_get_token_missing_section(self):
+        assert get_token({}) is None
+
+    def test_get_token_missing_key(self):
+        assert get_token({"joplin": {}}) is None
+
+    def test_get_map_path_present(self):
+        assert get_map_path({"projects": {"map": "/a/b.json"}}) == "/a/b.json"
+
+    def test_get_map_path_missing_section(self):
+        assert get_map_path({}) is None
+
+    def test_get_map_path_missing_key(self):
+        assert get_map_path({"projects": {}}) is None