feat(skill): unify skill activation as authorized tools

Expose skill tools (activate/register_skill/native exec) like native tools
instead of gating them behind the skill_authoring capability:
- toolmgr.get_all_tools drops include_skill_authoring; SkillToolLoader
  self-gates on sandbox + skill_mgr
- preproc drops the include_skill_authoring branch; pipeline-bound skills
  and the skills resource gate on skill_mgr presence

Persist activated skills into host.activated_skills conversation state so
they survive across runs (host writes at activate; last-write-wins); drop
the dead restore_activated_skills helper.

Prefill ToolResource.parameters host-side (tool_mgr.get_tool_schema) so
runners build LLM tools without per-tool get_tool_detail round-trips.

Align agent-runner-pluginization design docs to the all-tool model.

src/langbot/pkg/api/http/controller/groups/resources/tools.py

@@ -45,7 +45,7 @@ class ToolsRouterGroup(group.RouterGroup):
         @self.route('/<tool_name>', methods=['GET'], auth_type=group.AuthType.USER_TOKEN)
         async def _(tool_name: str) -> str:
             """获取特定工具详情"""
-            tools = await self.ap.tool_mgr.get_all_tools(include_skill_authoring=True)
+            tools = await self.ap.tool_mgr.get_all_tools()
 
             for tool in tools:
                 if tool.name == tool_name: