feat(mcp): add standalone server registry

Implement C.1 stdio MCP server CRUD, encrypted environment secrets, command digest approval, connection tests, lifecycle recovery, and dynamic tool registration. Add the standalone frontend configuration center, contracts, regression tests, and development documentation.

backend/app/container.py

@@ -5,6 +5,7 @@ from app.agent.builtin_tools import register_builtin_tools
 from app.contracts import ModelCapability, ProviderConfig, ProviderType
 from app.config import BACKEND_DIR, get_settings
 from app.extensions import PluginRuntime, SkillRuntime
+from app.extensions.mcp_registry import McpServerRegistry
 from app.providers import MockProvider, ProviderFactory, ProviderRegistry
 from app.providers.credentials import (
     ChainedCredentialResolver,
@@ -22,6 +23,7 @@ class ApplicationContainer:
     permissions: PermissionManager
     skills: SkillRuntime
     plugins: PluginRuntime
+    mcp_servers: McpServerRegistry
     agent: AgentRuntime
 
 
@@ -61,6 +63,14 @@ def build_container() -> ApplicationContainer:
     plugins.install(BACKEND_DIR / "extensions" / "plugins" / "text-tools")
     plugins.enable("text-tools")
 
+    mcp_servers = McpServerRegistry(
+        tools,
+        credentials,
+        settings.data_dir,
+        allow_process_launch=settings.environment == "development",
+    )
+    mcp_servers.restore_enabled()
+
     skills = SkillRuntime(tools)
     skills.install(BACKEND_DIR / "extensions" / "skills" / "knowledge-assistant")
     skills.enable("knowledge-assistant")
@@ -81,6 +91,7 @@ def build_container() -> ApplicationContainer:
         permissions=permissions,
         skills=skills,
         plugins=plugins,
+        mcp_servers=mcp_servers,
         agent=agent,
     )