feat: auto-install Python deps from template pyproject.toml via uv

ExternalToolManager.discover() now accepts template root dir, detects
pyproject.toml and runs `uv sync` to create a venv. Tool invocation and
schema discovery inject the venv PATH/VIRTUAL_ENV so template tools can
import declared dependencies without manual installation.

src/agent.rs

@@ -698,13 +698,13 @@ fn build_step_tools() -> Vec<Tool> {
             },
             "required": ["reason"]
         })),
-        make_tool("step_done", "完成当前步骤。必须提供摘要。可选声明本步骤的产出物。", serde_json::json!({
+        make_tool("step_done", "完成当前步骤。必须提供摘要和产出物列表（无产出物时传空数组）。", serde_json::json!({
             "type": "object",
             "properties": {
                 "summary": { "type": "string", "description": "本步骤的工作摘要" },
                 "artifacts": {
                     "type": "array",
-                    "description": "本步骤的产出物列表",
+                    "description": "本步骤的产出物列表。无产出物时传空数组 []",
                     "items": {
                         "type": "object",
                         "properties": {
@@ -717,7 +717,7 @@ fn build_step_tools() -> Vec<Tool> {
                     }
                 }
             },
-            "required": ["summary"]
+            "required": ["summary", "artifacts"]
         })),
         tool_kb_search(),
         tool_kb_read(),