Add MCP client plan

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

MCP_CLIENT_PLAN.md

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+# MCP Client Plan
+
+## Goal
+Add an MCP client mode to Yaak so users can connect to and debug MCP servers.
+
+## Core Design
+- **Protocol layer:** Implement JSON‑RPC framing, message IDs, and notifications as the common core.
+- **Transport interface:** Define an async trait with `connect`, `send`, `receive`, and `close` methods.
+- **Transports:**
+  - Start with **Standard I/O** for local development.
+  - Reuse the existing HTTP stack for **HTTP streaming** next.
+  - Leave hooks for **WebSocket** support later.
+
+## Integration
+- Register MCP as a new request type alongside REST, GraphQL, gRPC, and WebSocket.
+- Allow per‑request transport selection (stdio or HTTP).
+- Map inbound messages into a new MCP response model that feeds existing timeline and debug views.
+
+## Testing and Dog‑fooding
+- Convert Yaak's own MCP server to Standard I/O for local testing.
+- Use it internally to validate protocol behavior and message flow.
+- Add unit and integration tests for JSON‑RPC messaging and transport abstractions.
+
+## Future Refinements
+- Add WebSocket transport support once core paths are stable.
+- Extend timelines for protocol‑level visualization layered over raw transport events.
+- Implement version and capability negotiation between client and server.