test+docs(M4): tests get_lessons/get_homeworks + repli unique + doc sync

Tests:
- test_pronote_client : get_lessons/get_homeworks success + error
  propagation, ENT resolution, factory parent/student, mode dégradé
  messages/informations (ConnectionError, TimeoutError)
- test_fallback : repli unique ICAL→pronotepy et PRONOTEPY→iCal,
  filtre target_date, fetch_messages log + re-raise sans secrets,
  aucune source configurée → PipelineCriticalError
- 59 tests passent (16 client + 17 fallback + 26 iCal)

Documentation:
- TODO.md : 7 items M4 cochés, contrat repli corrigé (unique, non
  réciproque) selon décision D4
- GUIDE_DEV_PYTHON.md : get_agenda_fallback → get_lessons/get_homeworks,
  pronote_url/account_type ajoutés au modèle + env vars + .env.example,
  AgendaSource StrEnum, §5.1.8 fallback contract mis à jour (repli unique)

Co-authored-by: opencode/test-engineer <test-engineer@agents.invalid>
Co-authored-by: opencode/tech-writer <tech-writer@agents.invalid>

.secrets.baseline

@@ -139,11 +139,11 @@
         "type": "Hex High Entropy String",
         "filename": "GUIDE_DEV_PYTHON.md",
         "hashed_secret": "90bd1b48e958257948487b90bee080ba5ed00caa",
-        "is_secret": false,
         "is_verified": true,
-        "line_number": 5117
+        "line_number": 5124,
+        "is_secret": false
       }
     ]
   },
-  "generated_at": "2026-09-05T21:51:55Z"
+  "generated_at": "2026-09-06T13:36:29Z"
 }