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

nanobot/agent/tools/base.py

@@ -285,7 +285,6 @@ def tool_parameters(schema: dict[str, Any]) -> Callable[[type[_ToolT]], type[_To
         def parameters(self: Any) -> dict[str, Any]:
             return deepcopy(frozen)
 
-        cls._tool_parameters_schema = deepcopy(frozen)
         cls.parameters = parameters  # type: ignore[assignment]
 
         abstract = getattr(cls, "__abstractmethods__", None)