feat: add MCP client plugin (dsh-mcp-client)

Connects to an external MCP server and registers its tools on
ctx.tools. Supports stdio (child process) and Streamable HTTP
transports. Credential-shaped env vars are scrubbed before forwarding
to child processes.

- Plugin lifecycle: connect, sync tools, re-sync on ToolListChanged,
  dispose unregisters and closes
- Full JSDoc on all exports (@param/@returns on functions)
- 100% per-file coverage (apply lifecycle, args coercion, env scrubbing)
- Config catalog regenerated

docs/rfc/INDEX.md

@@ -12,6 +12,7 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
 | [Multiplex concurrent ACP sessions over one connection](proposed/feature/2026-06-14-acp-multi-session.md) | 2026-06-14 |
 | [Optional Code Mode — model writes TypeScript against an SDK of all tools](proposed/feature/2026-06-15-optional-code-mode.md) | 2026-06-15 |
 | [Pre-tool input rewrite — a consistent design](proposed/feature/2026-06-30-pre-tool-input-rewrite.md) | 2026-06-30 |
+| [MCP client plugin — connect to external MCP servers and bridge their tools](proposed/feature/2026-07-07-mcp-client-plugin.md) | 2026-07-07 |
 
 ### Simplification