chore: remove dead code identified by vulture + coverage cross-validation

Remove unused code confirmed dead via vulture scan, grep verification,
and coverage analysis:

- _get_bridge_dir (cli/commands.py): 82-line function with zero callers
- add_assistant_message (agent/context.py): method body never executed,
  also removed now-unused build_assistant_message import
- _tool_parameters_schema (agent/tools/base.py): redundant copy of schema
  already exposed via the `parameters` property
- MSTEAMS_REF_TTL_S (channels/msteams.py): unused constant (production
  uses config.ref_ttl_days directly); inlined in test
- MESSAGE_TYPE_USER (channels/weixin.py): unused constant

tests/test_msteams.py

@@ -169,7 +169,7 @@ def test_init_prunes_stale_and_unsupported_conversation_refs(make_channel, tmp_p
                 "conv-valid": {"updated_at": now - 60},
                 "conv-webchat": {"updated_at": now - 60},
                 "conv-group": {"updated_at": now - 60},
-                "conv-stale": {"updated_at": now - msteams_module.MSTEAMS_REF_TTL_S - 1},
+                "conv-stale": {"updated_at": now - 30 * 24 * 60 * 60 - 1},
             },
             indent=2,
         ),