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.

frontend/src/contracts/index.ts

@@ -193,7 +193,7 @@ export interface ToolDefinition {
   name: string
   description: string
   parameters: Record<string, unknown>
-  source?: 'builtin' | 'plugin'
+  source?: 'builtin' | 'plugin' | 'mcp_server'
   plugin_id?: string
 }
 
@@ -535,6 +535,38 @@ export interface OperationResponse {
   message?: string | null
 }
 
+export type McpServerTransport = 'stdio' | 'streamable_http' | 'sse'
+export type McpServerState = 'stopped' | 'starting' | 'ready' | 'unhealthy' | 'error'
+
+export interface McpServerInput {
+  name: string
+  transport: McpServerTransport
+  command: string
+  args: string[]
+  environment: Record<string, string>
+  secret_environment_keys: string[]
+  permissions: string[]
+  startup_timeout_seconds: number
+  tool_timeout_seconds: number
+}
+
+export interface McpServer extends Omit<McpServerInput, 'secret_environment_keys'> {
+  server_id: string
+  secret_environment: Record<string, boolean>
+  enabled: boolean
+  trusted: boolean
+  command_digest: string
+  command_summary: string
+  status: McpServerState
+  tools_count: number
+  protocol_version?: string | null
+  remote_server_name?: string | null
+  remote_server_version?: string | null
+  error?: string | null
+  last_tested_at?: string | null
+  last_test_succeeded?: boolean | null
+}
+
 export interface ApiNoteBlock {
   block_id: string
   note_id: string