fix(M10): apply FIXME_M10 corrections (transport, dry_run, format, security)

Fix all 8 findings from the independent review (FIXME_M10.md):

#1 Transport compatible with slixmpp 1.17.0 (D5):
  - Use real ClientXMPP type (remove Any), JID with resource
  - connect(host, port) explicit, no use_tls kwarg
  - enable_direct_tls/enable_starttls configured before connect
  - Single timeout via asyncio.Future for session_start/failed_auth/disconnected
  - Remove premature 'starttls' in features check, remove auto_reconnect
  - try/finally guarantees disconnect on all paths (#4)

#2 Factory dry_run no longer bypassed (D6):
  - Single send() entry point in SyncXmppChannel
  - dry_run check before any ClientXMPP creation
  - Remove XmppChannel.send() dual implementation

#3 Thread daemon removed — single asyncio.run(), documented limitation

#5 Richer message format:
  - Target date header, change type [Ajouté/Supprimé/Modifié]
  - Lesson times, homework due date, message author
  - No pronote_messages duplication (external_info = blog + other_info only)

#6 Error contract unified (D6):
  - Channel.send() -> bool never raises PipelineWarning
  - Errors logged with redaction, returns False
  - PipelineWarning(step='xmpp') will be created by pipeline M11

#7 Tests faithful to slixmpp 1.17.0 API:
  - FakeClientXMPP with real connect(host,port)/disconnect() signatures
  - Assertions on host, port, resource, mtype='chat'
  - No RuntimeWarning from unawaited coroutines

#8 .secrets.baseline restored from main

Coverage: 96.44% on channels/, 600 tests pass, pre-commit all-files green.

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

pronote_sync/channels/protocol.py

@@ -20,6 +20,12 @@ class Channel(Protocol):
     def send(self, message: XmppMessage) -> bool:
         """Envoie un message sur le canal.
 
+        Un canal ne lève jamais :pyexc:`PipelineWarning` ; en cas d'échec, il
+        retourne ``False``. Le :pyexc:`PipelineWarning` est créé par l'étape
+        pipeline, pas par le canal. Une :pyexc:`PipelineCriticalError` peut
+        en revanche être levée en cas de panne critique (ex. : chemin
+        CalDAV, non utilisé par le canal XMPP).
+
         :param message: Message final à transmettre.
         :return: ``True`` si l'envoi a réussi, ``False`` sinon.
         :rtype: bool