Expose MCP resources to Codex agents

Summary

Codex’s MCP integration surfaces tool wrappers end-to-end, but resources announced during the MCP handshake never reach the CLI agent. The client layer only issues tools/list and tools/call, so resources are invisible to the runtime and UI. This gap prevents agents from reading MCP-provided configuration or documents automatically.

Proposed design

1. Add resource helpers in the client. Extend mcp-client/rmcp-client with typed calls for resources/list, resources/read, resources/subscribe, and resources/unsubscribe, mirroring the existing tool helpers and wiring notifications into the dispatcher.
2. Cache resources in the connection manager. Teach McpConnectionManager to fetch resources alongside tools during startup, maintain a per-server resource cache, and react to resource update notifications so the data stays current.
3. Expose resources via protocol & UI. Introduce new protocol messages (e.g., Op::ListMcpResources, EventMsg::McpListResourcesResponse) so front-ends can request and render resource listings. Update the TUI to display them and optionally add a convenience path for triggering resources/read.
4. Agent runtime integration. Provide a higher-level API that lets plans fetch resource://… URIs directly, similar to the existing tool wrappers, so agents can ingest resource content without manual intervention.

Acceptance criteria

• Agents can enumerate MCP resources from a running session and read their contents without leaving the CLI.
• Resource caches stay in sync when servers send notifications/resources/* updates.
• Documentation notes the new capability and how MCP authors should expect their resources to appear.

Happy to iterate if there’s a preferred rollout path.