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.

src/timesheets/joplin.py

@@ -0,0 +1,68 @@
+"""
+Joplin integration via the joppy ClientApi.
+
+Actual notebook structure:
+    Work > Timesheets > YYYY
+
+Notes live directly inside the year notebook and are titled 'YYYY - WNN'.
+"""
+
+from datetime import date
+from typing import Optional
+
+from joppy.client_api import ClientApi
+
+
+def _iso_week_label(d: date) -> str:
+    """Return the note title for the week containing the given date, e.g. '2026 - W21'."""
+    year, week, _ = d.isocalendar()
+    return f"{year} - W{week:02d}"
+
+
+def _find_notebook(
+    api: ClientApi, title: str, parent_id: Optional[str] = None
+) -> Optional[str]:
+    """Return the ID of a notebook matching title (and optionally parent_id), or None."""
+    for nb in api.get_all_notebooks():
+        if nb.title == title:
+            if parent_id is None or nb.parent_id == parent_id:
+                return nb.id
+    return None
+
+
+def fetch_week_note(token: str, target_date: date) -> str:
+    """
+    Fetch the body of the weekly timesheet note from Joplin for the week
+    containing target_date.
+
+    Notebook path: Work > Timesheets > YYYY
+    Note title:    YYYY - WNN
+
+    Raises RuntimeError if any notebook or the note cannot be found.
+    """
+    api = ClientApi(token=token)
+    week_label = _iso_week_label(target_date)
+    year_str = str(target_date.year)
+
+    work_id = _find_notebook(api, "Work")
+    if work_id is None:
+        raise RuntimeError("Joplin notebook 'Work' not found")
+
+    timesheets_id = _find_notebook(api, "Timesheets", parent_id=work_id)
+    if timesheets_id is None:
+        raise RuntimeError("Joplin notebook 'Work > Timesheets' not found")
+
+    year_id = _find_notebook(api, year_str, parent_id=timesheets_id)
+    if year_id is None:
+        raise RuntimeError(
+            f"Joplin notebook 'Work > Timesheets > {year_str}' not found"
+        )
+
+    notes = api.get_all_notes(notebook_id=year_id, fields="id,title,body")
+    for note in notes:
+        if note.title == week_label:
+            return note.body
+
+    raise RuntimeError(
+        f"Joplin note '{week_label}' not found in 'Work > Timesheets > {year_str}'"
+    )