refactor(tools): unify tool-detail normalization in ToolManager

Drop the PluginToolLoader.get_tool() override that returned a raw
ComponentManifest, so every loader's get_tool() now returns a uniform
resource_tool.LLMTool (PluginToolLoader.get_tools() already did this
conversion). This removes the only source of tool-shape heterogeneity.

- ToolManager.get_tool_schema(): drop the ComponentManifest-vs-LLMTool branch
- ToolManager.get_tool_detail(): new host-level shape {name, description,
  human_desc, parameters}
- handler.py GET_TOOL_DETAIL: call tool_mgr.get_tool_detail(); delete the
  handler-local _build_tool_detail + _i18n_to_dict/_i18n_to_text adapters and
  the litellm TODO
- ToolLookupResult is now just LLMTool

The dropped label/spec fields were not consumed by any runner (local-agent
build_llm_tool and external harnesses use only name/description/parameters).

src/langbot/pkg/provider/tools/loaders/plugin.py

@@ -3,7 +3,6 @@ from __future__ import annotations
 import typing
 import traceback
 
-from langbot_plugin.api.definition.components.manifest import ComponentManifest
 from langbot_plugin.api.entities.events import pipeline_query
 
 from .. import loader
@@ -58,12 +57,6 @@ class PluginToolLoader(loader.ToolLoader):
                 return True
         return False
 
-    async def get_tool(self, name: str) -> ComponentManifest | None:
-        for tool in await self.ap.plugin_connector.list_tools():
-            if tool.metadata.name == name:
-                return tool
-        return None
-
     async def invoke_tool(self, name: str, parameters: dict, query: pipeline_query.Query) -> typing.Any:
         try:
             return await self.ap.plugin_connector.call_tool(