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.

AGENTS.md

@@ -4,7 +4,7 @@
 
 A Python CLI tool that parses markdown pipe-delimited timesheet tables and
 exports them to CSV for import into Odoo (or similar tools). It also supports
-a human-readable summary view.
+a human-readable summary view and can fetch notes directly from Joplin.
 
 ### Package layout
 
@@ -14,9 +14,10 @@ timesheets/
 ├── AGENTS.md
 └── src/timesheets/
     ├── cli.py        # argument parsing, main() entry point
-    ├── parser.py     # markdown table parsing and row aggregation
+    ├── parser.py     # markdown table parsing, aggregation, date filtering
     ├── projects.py   # project_map.json loading and key resolution
     ├── output.py     # CSV writing and summary printing
+    ├── joplin.py     # Joplin API integration (notebook traversal, note fetching)
     └── utils.py      # shared low-level helpers (duration parsing, formatting, etc.)
 ```
 
@@ -27,7 +28,8 @@ tests/
 ├── test_utils.py
 ├── test_parser.py
 ├── test_projects.py
-└── test_output.py
+├── test_output.py
+└── test_joplin.py
 ```
 
 ---
@@ -57,7 +59,7 @@ uv run timesheets input.md
 uv run timesheets input.md -o output.csv
 
 # Override the date (DD/MM/YY)
-uv run timesheets input.md --date 22/03/26
+uv run timesheets input.md --date 22/05/26
 
 # Use a specific project map file
 uv run timesheets input.md --map /path/to/project_map.json
@@ -67,13 +69,44 @@ uv run timesheets input.md --summary
 
 # Read from stdin
 cat input.md | uv run timesheets -
+
+# Fetch today's entries from Joplin (token via env var)
+JOPLIN_TOKEN=your_token uv run timesheets --joplin
+
+# Fetch entries for a specific date from Joplin
+uv run timesheets --joplin --date 22/05/26 --token your_token
 ```
 
+The `--joplin` flag and the file `input` argument are mutually exclusive.
+When `--joplin` is used, only entries matching the target date (from `--date`,
+or today) are returned, filtered by the `# ... YYYY-MM-DD` day heading in the note.
+
+The API token can be provided via:
+- `--token <token>` CLI flag
+- `JOPLIN_TOKEN` environment variable
+
 `project_map.json` is auto-discovered in the current working directory if
 `--map` is not provided.
 
 ---
 
+## Joplin notebook structure
+
+The `--joplin` flag expects the following notebook hierarchy in Joplin:
+
+```
+Work/
+└── Timesheets/
+    └── YYYY/
+        └── YYYY - WNN/         ← notebook per week
+            └── YYYY - WNN      ← note with the same title as the notebook
+```
+
+The note body contains one markdown table per day, each preceded by a heading
+of the form `# <weekday> - YYYY-MM-DD`.
+
+---
+
 ## Testing
 
 The test suite uses **pytest** with **pytest-cov** for coverage reporting.
@@ -110,3 +143,6 @@ uv run pytest tests/test_parser.py::TestParseTable::test_empty_input
 
 4. `cli.py` is intentionally excluded from unit tests — it is thin glue code.
    All logic worth testing belongs in the other modules.
+
+5. Joplin integration tests in `test_joplin.py` must mock `ClientApi` — do not
+   require a live Joplin instance.