feat(joplin): add --joplin flag to fetch weekly timesheet note from Joplin

- Add joplin.py with fetch_week_note() that walks Work > Timesheets > YYYY
  and returns the body of the matching YYYY-WNN note via joppy ClientApi
- Add filter_rows_by_date() to parser.py to extract only rows belonging
  to a specific day based on '# ... YYYY-MM-DD' headings in the document
- Update cli.py: input and --joplin are now a mutually exclusive required
  group; add --token flag with JOPLIN_TOKEN env var fallback; --date is
  parsed into a real date object used for both output and day filtering
- Add joppy as a runtime dependency (lazy-imported in cli.py)
- Add tests for filter_rows_by_date and full mocked coverage of joplin.py
- Update AGENTS.md with Joplin usage, notebook structure, and test rules

The actual Joplin structure has notes directly inside the year notebook
(Work > Timesheets > YYYY), not in per-week sub-notebooks as initially
assumed. fetch_week_note() reflects this flat structure.

tests/test_joplin.py

@@ -0,0 +1,116 @@
+"""
+Tests for the Joplin integration module.
+
+All tests mock the joppy ClientApi so no live Joplin instance is required.
+"""
+
+from datetime import date
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from timesheets.joplin import _iso_week_label, fetch_week_note
+
+# ---------------------------------------------------------------------------
+# _iso_week_label
+# ---------------------------------------------------------------------------
+
+
+class TestIsoWeekLabel:
+    @pytest.mark.parametrize(
+        "d, expected",
+        [
+            (date(2026, 5, 22), "2026 - W21"),
+            (date(2026, 5, 18), "2026 - W21"),  # Monday of same week
+            (date(2026, 1, 1), "2026 - W01"),
+            (date(2026, 12, 28), "2026 - W53"),
+        ],
+    )
+    def test_label(self, d, expected):
+        assert _iso_week_label(d) == expected
+
+
+# ---------------------------------------------------------------------------
+# fetch_week_note — mocked API
+# ---------------------------------------------------------------------------
+
+
+def _make_notebook(id_, title, parent_id=""):
+    nb = MagicMock()
+    nb.id = id_
+    nb.title = title
+    nb.parent_id = parent_id
+    return nb
+
+
+def _make_note(id_, title, body):
+    note = MagicMock()
+    note.id = id_
+    note.title = title
+    note.body = body
+    return note
+
+
+def _build_api(note_body: str, week_label: str = "2026 - W21"):
+    """Return a mocked ClientApi that serves a minimal notebook tree."""
+    api = MagicMock()
+    api.get_all_notebooks.return_value = [
+        _make_notebook("work_id", "Work", parent_id=""),
+        _make_notebook("ts_id", "Timesheets", parent_id="work_id"),
+        _make_notebook("yr_id", "2026", parent_id="ts_id"),
+    ]
+    api.get_all_notes.return_value = [
+        _make_note("note_id", week_label, note_body),
+    ]
+    return api
+
+
+class TestFetchWeekNote:
+    def _patch(self, api):
+        return patch("timesheets.joplin.ClientApi", return_value=api)
+
+    def test_returns_note_body(self):
+        api = _build_api("# content")
+        with self._patch(api):
+            result = fetch_week_note("dummy_token", date(2026, 5, 22))
+        assert result == "# content"
+
+    def test_constructs_client_with_token(self):
+        api = _build_api("body")
+        with patch("timesheets.joplin.ClientApi", return_value=api) as mock_cls:
+            fetch_week_note("my_secret_token", date(2026, 5, 22))
+        mock_cls.assert_called_once_with(token="my_secret_token")
+
+    def test_missing_work_notebook_raises(self):
+        api = MagicMock()
+        api.get_all_notebooks.return_value = []
+        with self._patch(api):
+            with pytest.raises(RuntimeError, match="'Work' not found"):
+                fetch_week_note("token", date(2026, 5, 22))
+
+    def test_missing_timesheets_notebook_raises(self):
+        api = MagicMock()
+        api.get_all_notebooks.return_value = [
+            _make_notebook("work_id", "Work", parent_id=""),
+        ]
+        with self._patch(api):
+            with pytest.raises(RuntimeError, match="Timesheets"):
+                fetch_week_note("token", date(2026, 5, 22))
+
+    def test_missing_year_notebook_raises(self):
+        api = MagicMock()
+        api.get_all_notebooks.return_value = [
+            _make_notebook("work_id", "Work", parent_id=""),
+            _make_notebook("ts_id", "Timesheets", parent_id="work_id"),
+        ]
+        with self._patch(api):
+            with pytest.raises(RuntimeError, match="2026"):
+                fetch_week_note("token", date(2026, 5, 22))
+
+    def test_missing_note_raises(self):
+        api = _build_api("body", week_label="2026 - W21")
+        # Return a note with the wrong title
+        api.get_all_notes.return_value = [_make_note("x", "wrong title", "body")]
+        with self._patch(api):
+            with pytest.raises(RuntimeError, match="note '2026 - W21' not found"):
+                fetch_week_note("token", date(2026, 5, 22))