feat(client): add connectClientMcp helper for client-hosted MCP tools - #57504

Stack 3/5, based on client-mcp-attach. Merge in order.

Part of the client MCP stack (experimental): a client (TUI/desktop/web) opens a WebSocket to the server, and the server treats that incoming socket as an internal MCP server scoped to a location. Tool calls carry _meta["ai.opencode/sessionID"] so the client acts on the calling session. MCP has no official WebSocket transport, so this is a custom transport, which the spec allows.

This PR

Adds @opencode/client/mcp, a separate subpath that doesn't import core or server and has no runtime dependencies:

• connectClientMcp({ url, name, directory?, headers?, tools }) opens the WebSocket, serves the tools, and reconnects with backoff (0.5s → 10s, reset once the server sends notifications/initialized)
• Tools are a plain array: { name, description?, inputSchema? (JSON Schema), handler(args, ctx) }. ctx.sessionID holds the calling session
• The MCP server is a small hand-written JSON-RPC handler, not @modelcontextprotocol/server. It only answers what core's attached MCP client uses: initialize (echoes the requested legacy protocol version), ping, tools/list and tools/call. Unknown tools get -32602, unknown methods get -32601, a handler that throws returns an isError: true result, and malformed frames are ignored
• Arguments aren't validated here because core already validates tool input against inputSchema before calling

Using a hand-written server instead of the SDK and zod shrinks the minified browser bundle of @opencode/client/mcp from 251 KB (64 KB gzip) to 2.1 KB (1.1 KB gzip). It also keeps them out of every @opencode/client install. @modelcontextprotocol/client is only a dev dependency.

Tests run a Bun WebSocket server acting as the opencode server, and the real MCP SDK client talks to the hand-written server. They cover the handshake, listing, calls with and without a session ID, errors, reconnects and backoff.

Stack

1. feat(core): add MCP transport over incoming client sockets #57502 feat(core): add MCP transport over incoming client sockets
2. feat(core): attach client-hosted MCP servers #57503 feat(core): attach client-hosted MCP servers
   → 3. feat(client): add connectClientMcp helper for client-hosted MCP tools #57504 feat(client): add connectClientMcp helper for client-hosted MCP tools
3. feat(server): add client MCP websocket endpoint (experimental) #57505 feat(server): add client MCP websocket endpoint (experimental)
4. feat(tui): expose open_terminal over client MCP (experimental) #57506 feat(tui): expose open_terminal over client MCP (experimental)