Add OpenAI tool-calling agentic loop with file tools and MiniMax MCP integration

- AIService.generate() now runs an agentic loop: sends tools to the API,
  handles tool_calls responses, executes tools, appends results, and repeats
  until a text response (up to tool_max_rounds, default 10)
- New app/ai/tools.py: six sandboxed file tools (write/read/create/modify/delete/list),
  OpenAI JSON schemas, path sandbox validator, and MCP→OpenAI schema converter
- Paths are sandboxed to workspace/ (project root) and prompts/ai/; traversal blocked
- MinimaxImageUnderstandClient gains list_tools() and call_tool() methods; MCP tool
  schemas are lazily fetched once per session and forwarded to the AI as callable tools
- AIConfig gains workspace_dir (Path, default workspace/) and tool_max_rounds (int, default 10)
- MCP client opened lazily only when an MCP tool is actually called during a loop

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

app/config.py

@@ -71,6 +71,8 @@ class AIConfig:
     image_understand: AIImageUnderstandConfig | None = None
     providers: dict[str, AIProviderConfig] = field(default_factory=dict)
     models: dict[str, AIModelConfig] = field(default_factory=dict)
+    workspace_dir: Path = field(default_factory=lambda: Path("workspace"))
+    tool_max_rounds: int = 10
 
 
 @dataclass(slots=True)
@@ -289,6 +291,15 @@ def _parse_ai_config(raw_ai: Any, *, root: Path) -> AIConfig | None:
                 f"ai.models.{alias}.provider references unknown provider: {model_config.provider}"
             )
 
+    workspace_dir_raw = raw_ai.get("workspace_dir", "workspace")
+    workspace_dir = Path(str(workspace_dir_raw)).expanduser()
+    if not workspace_dir.is_absolute():
+        workspace_dir = (root / workspace_dir).resolve()
+
+    tool_max_rounds = _parse_int(raw_ai.get("tool_max_rounds", 10), field_name="ai.tool_max_rounds")
+    if tool_max_rounds < 1:
+        raise ValueError("ai.tool_max_rounds must be >= 1")
+
     return AIConfig(
         default_model=default_model,
         context_messages=context_messages,
@@ -296,6 +307,8 @@ def _parse_ai_config(raw_ai: Any, *, root: Path) -> AIConfig | None:
         image_understand=image_understand,
         providers=providers,
         models=models,
+        workspace_dir=workspace_dir,
+        tool_max_rounds=tool_max_rounds,
     )