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.

pyproject.toml

@@ -7,7 +7,9 @@ authors = [
     { name = "Jef Roosens", email = "roosenj@factry.io" }
 ]
 requires-python = ">=3.13"
-dependencies = []
+dependencies = [
+    "joppy>=1.0.2",
+]
 
 [project.scripts]
 timesheets = "timesheets.cli:main"