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>

.secrets.baseline

@@ -133,49 +133,6 @@
       ]
     }
   ],
-  "results": {
-    "GUIDE_DEV_PYTHON.md": [
-      {
-        "type": "Hex High Entropy String",
-        "filename": "GUIDE_DEV_PYTHON.md",
-        "hashed_secret": "90bd1b48e958257948487b90bee080ba5ed00caa",
-        "is_verified": true,
-        "line_number": 4935,
-        "is_secret": false
-      }
-    ],
-    "tests/unit/test_caldav_gateway.py": [
-      {
-        "type": "Secret Keyword",
-        "filename": "tests/unit/test_caldav_gateway.py",
-        "hashed_secret": "1c58bd92003bbaa0538e249fff6ee19a270dec5f",
-        "is_verified": false,
-        "line_number": 152
-      },
-      {
-        "type": "Basic Auth Credentials",
-        "filename": "tests/unit/test_caldav_gateway.py",
-        "hashed_secret": "1c58bd92003bbaa0538e249fff6ee19a270dec5f",
-        "is_verified": false,
-        "line_number": 763
-      }
-    ],
-    "tests/unit/test_caldav_security.py": [
-      {
-        "type": "Basic Auth Credentials",
-        "filename": "tests/unit/test_caldav_security.py",
-        "hashed_secret": "8e1f07a2939b6324c70f48a3e7f64b463a4a3f8b",
-        "is_verified": false,
-        "line_number": 27
-      },
-      {
-        "type": "Secret Keyword",
-        "filename": "tests/unit/test_caldav_security.py",
-        "hashed_secret": "6b554cd7b7e0115065fb4907307a74f1902154d4",
-        "is_verified": false,
-        "line_number": 28
-      }
-    ]
-  },
-  "generated_at": "2026-09-07T17:59:08Z"
+  "results": {},
+  "generated_at": "2026-09-07T21:10:53Z"
 }