feat(agent): add external MCP client support

Reuse OpenCode 2 HTTP/stdio clients with per-session read-only grants,
native approval and sanitized owner-scoped MCP status. Narrow the
environment inherited by trusted local MCP processes.

Validated in an isolated checkout with bun run check and 20 focused tests.

tests/frontendAction/callIdPlugin.test.ts

@@ -7,7 +7,8 @@ describe("OpenCode 2 business tool registration", () => {
     const storage = new Map<string, unknown>();
     let rpcList: (() => Promise<unknown>) | undefined;
     await plugin.setup({
-      tool: { transform: async (fn: (editor: unknown) => void) => fn({ add: (tool: any) => tools.set(tool.name, tool), remove: (name: string) => tools.delete(name), update: () => {} }), list: async () => Array.from(tools, ([id, tool]) => ({ ...tool, id })) },
+      tool: { transform: async (fn: (editor: unknown) => void) => fn({ list: () => Array.from(tools, ([id, tool]) => ({ ...tool, id })), add: (tool: any) => tools.set(tool.name, tool), remove: (name: string) => tools.delete(name), update: () => {} }), list: async () => Array.from(tools, ([id, tool]) => ({ ...tool, id })) },
+      mcp: { transform: async (fn: (editor: unknown) => void) => fn({ list: () => [] }), list: async () => ({ data: [] }) },
       shell: { hook: async () => {} },
       permission: { hook: async () => {} },
       session: { hook: async () => {} },