feat(tools): add tool_search for deferred MCP tool loading (#1176)

* feat(tools): add tool_search for deferred MCP tool loading

When multiple MCP servers are enabled, total tool count can exceed 30-50,
causing context bloat and degraded tool selection accuracy. This adds a
deferred tool loading mechanism controlled by `tool_search.enabled` config.

- Add ToolSearchConfig with single `enabled` field
- Add DeferredToolRegistry with regex search (select:, +keyword, keyword)
- Add tool_search tool returning OpenAI-compatible function JSON
- Add DeferredToolFilterMiddleware to hide deferred schemas from bind_tools
- Add <available-deferred-tools> section to system prompt
- Enable MCP tool_name_prefix to prevent cross-server name collisions
- Add 34 unit tests covering registry, tool, prompt, and middleware

* fix: reset stale deferred registry and bump config_version

- Reset deferred registry upfront in get_available_tools() to prevent
  stale tool entries when MCP servers are disabled between calls
- Bump config_version to 2 for new tool_search config field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(tests): mock get_app_config in prompt section tests for CI

CI has no config.yaml, causing TestDeferredToolsPromptSection to fail
with FileNotFoundError. Add autouse fixture to mock get_app_config.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/packages/harness/deerflow/config/app_config.py

@@ -17,6 +17,7 @@ from deerflow.config.subagents_config import load_subagents_config_from_dict
 from deerflow.config.summarization_config import load_summarization_config_from_dict
 from deerflow.config.title_config import load_title_config_from_dict
 from deerflow.config.tool_config import ToolConfig, ToolGroupConfig
+from deerflow.config.tool_search_config import ToolSearchConfig, load_tool_search_config_from_dict
 
 load_dotenv()
 
@@ -32,6 +33,7 @@ class AppConfig(BaseModel):
     tool_groups: list[ToolGroupConfig] = Field(default_factory=list, description="Available tool groups")
     skills: SkillsConfig = Field(default_factory=SkillsConfig, description="Skills configuration")
     extensions: ExtensionsConfig = Field(default_factory=ExtensionsConfig, description="Extensions configuration (MCP servers and skills state)")
+    tool_search: ToolSearchConfig = Field(default_factory=ToolSearchConfig, description="Tool search / deferred loading configuration")
     model_config = ConfigDict(extra="allow", frozen=False)
     checkpointer: CheckpointerConfig | None = Field(default=None, description="Checkpointer configuration")
 
@@ -101,6 +103,10 @@ class AppConfig(BaseModel):
         if "subagents" in config_data:
             load_subagents_config_from_dict(config_data["subagents"])
 
+        # Load tool_search config if present
+        if "tool_search" in config_data:
+            load_tool_search_config_from_dict(config_data["tool_search"])
+
         # Load checkpointer config if present
         if "checkpointer" in config_data:
             load_checkpointer_config_from_dict(config_data["checkpointer"])