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_parser.py

@@ -7,6 +7,7 @@ from timesheets.parser import (
     build_description,
     detect_has_duration_column,
     extract_table_blocks,
+    filter_rows_by_date,
     parse_document,
     parse_table,
 )
@@ -215,6 +216,67 @@ class TestParseDocument:
             )
 
 
+# ---------------------------------------------------------------------------
+# filter_rows_by_date
+# ---------------------------------------------------------------------------
+
+
+class TestFilterRowsByDate:
+    # Reuse the W21 file which has one table per day-heading
+    with open(WEEK_FILE, encoding="utf-8") as _f:
+        _WEEK_LINES = _f.read().splitlines()
+
+    def test_returns_only_matching_day(self):
+        from datetime import date
+
+        rows = filter_rows_by_date(self._WEEK_LINES, date(2026, 5, 22))
+        assert len(rows) > 0
+        # Friday has these projects in the sample file
+        projects = {r["project"] for r in rows}
+        assert "scrum" in projects
+
+    def test_different_day_returns_different_rows(self):
+        from datetime import date
+
+        rows_fri = filter_rows_by_date(self._WEEK_LINES, date(2026, 5, 22))
+        rows_mon = filter_rows_by_date(self._WEEK_LINES, date(2026, 5, 18))
+        assert rows_fri != rows_mon
+        assert len(rows_mon) > 0
+
+    def test_no_match_returns_empty(self):
+        from datetime import date
+
+        rows = filter_rows_by_date(self._WEEK_LINES, date(2026, 1, 1))
+        assert rows == []
+
+    def test_inline_document(self):
+        from datetime import date
+
+        lines = [
+            "# Maandag - 2026-05-18",
+            "| Start | End   | Project | Story | Note |",
+            "|-------|-------|---------|-------|------||",
+            "| 08:00 | 08:30 | bugs    |       | fix  |",
+            "",
+            "# Dinsdag - 2026-05-19",
+            "| Start | End   | Project | Story | Note |",
+            "|-------|-------|---------|-------|------||",
+            "| 09:00 | 09:30 | scrum   |       | dsu  |",
+        ]
+        rows = filter_rows_by_date(lines, date(2026, 5, 18))
+        assert len(rows) == 1
+        assert rows[0]["project"] == "bugs"
+
+        rows = filter_rows_by_date(lines, date(2026, 5, 19))
+        assert len(rows) == 1
+        assert rows[0]["project"] == "scrum"
+
+    def test_empty_input(self):
+        from datetime import date
+
+        assert filter_rows_by_date([], date(2026, 5, 22)) == []
+
+
 # ---------------------------------------------------------------------------
 # build_description
 # ---------------------------------------------------------------------------