feat(M13): complete test fixtures, shared conftest, and coverage >= 90%

Finalize M13 test and coverage milestone:

tests/fixtures/pronote-6e.ics (new):
- Anonymized iCal fixture for Classe de 6e (3 VEVENTs: SVT lesson with
  homework block, Histoire-Géo modified lesson, all-day school outing)
- Same structure as pronote-4e.ics, no secrets or real data

tests/conftest.py:
- Added sample_message fixture (Message with MessageType.INFORMATION)
- Integrated sample_message into pronote_data fixture (messages=[sample_message])
- Sphinx/reST docstring with :return: and :rtype:

.gitignore:
- Fixed typo: .worktress/ -> .worktrees/ (line 56)

pyproject.toml:
- Added ".worktrees" to ruff extend-exclude to prevent ruff format --check .
  from scanning worktree files

TODO.md:
- Checked M13 items: tests/fixtures/ and tests/conftest.py

Validation: 627 tests pass, coverage 95.67% (threshold 90%), ruff/mypy/
bandit/pre-commit all green.

Co-authored-by: opencode/coder <coder@agents.invalid>

.gitignore

@@ -53,7 +53,7 @@ Thumbs.db
 # --- Local scratch / WIP files ---
 FIXME_*
 TEST_*
-.worktress/
+.worktrees/
 
 # --- Logs ---
 *.log