feat: implement XmppChannel and SyncXmppChannel (M10-U4+U5)

XmppChannel sends direct messages via slixmpp ClientXMPP with:
- _format_message: 5 emoji sections (synthèse, agenda, devoirs, messages, infos)
  with sanitize_plaintext on all content (SEC-XMPP-06)
- send_async: public async method with connect, STARTTLS verification,
  send_message, disconnect lifecycle (D1, SEC-XMPP-04)
- send: sync wrapper via asyncio.run() for direct callers

SyncXmppChannel adapts async XmppChannel for synchronous pipeline use (D4):
- asyncio.run() when no event loop running (nominal pipeline)
- daemon thread with timeout when event loop already running
- Returns False on any error, never raises (non-blocking)

Security:
- auto_reconnect=False, failed_auth → disconnect + PipelineWarning (SEC-XMPP-04)
- __cause__ and __context__ cleared on all PipelineWarning raises (SEC-XMPP-05)
- redact_secrets with extra_secrets=[jid, password, to] on all logs (SEC-XMPP-02)
- STARTTLS features check post-connection, disconnect on failure (D1)
- dry-run mode logs redacted message without connecting

28 unit tests (20 channel + 8 adapter) covering success, errors, security.

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

pyproject.toml

@@ -121,6 +121,10 @@ strict = true
 module = "litellm"
 ignore_missing_imports = true
 
+[[tool.mypy.overrides]]
+module = "slixmpp"
+ignore_missing_imports = true
+
 [[tool.mypy.overrides]]
 module = "openai.*"
 follow_imports = "skip"