feat(control): expose external MCP through an isolated capability SDK #793

External operators currently have to navigate Agent Code through a complex UI and keyboard surface. Add an opt-in external MCP server that complements computer use with precise, broad common controls and a complete app reference.

The approved architecture is a small internal SDK for feature-owned registration and invocation. Handlers reuse domain operations. MCP is a removable outer adapter; ordinary features do not import protocol types or tool names.

First-release acceptance criteria (scope refined by the user's request to ship and merge the initial state):

• Search/list commands and effective/default/fixed/contextual keybindings; describe the app and all shipped features, including UI-only routes.
• Open the real command picker with acknowledged query/selection and support computer-use handoffs.
• Stable multi-window ownership and exact agent navigation; create/prompt/read agents, manage drafts and advertised conditions, adjust layout/Dispatch, open editor files, create/read/write terminals, and request normal confirmed agent close.
• Lightweight conversation reads default to actual user prompts and all visible assistant prose; separate depth/range, immutable pagination, deltas and full payload continuation.
• Durable retrievable invocation history with deduplication and explicit unknown outcomes; long close operations expose durable completion separately from acceptance.
• Settings enables an app-wide loopback MCP listener, disabled by default, and installs/updates/removes the app-owned global Codex connection and Codex-only operator skill. Preserve unrelated user configuration and report edited/unmanaged conflicts.
• External Codex clients sharing that Codex home receive both. Agent Code's internal launches/resume/recovery and workflow workers explicitly exclude the reserved operator MCP and skill. Global settings changes may require restarting the external client.
• Enforced SDK/MCP import boundaries, relevant real provider evidence, actual two-window Electron/MCP verification, and production build/output verification. No general recording harness prerequisite.

Deferred tool families remain in the product plan: richer provider switching/rewind/recovery, bulk tasks, settings/template edits and workflow orchestration. Existing UI/command routes remain usable through computer use. This initial-release issue does not promise every planned tool family or cloud-hosted ChatGPT connectivity.

Plan: docs/superpowers/plans/2026-09-04-external-control-mcp.md.
Decomposition and verification: docs/decomposition/external-control-sdk.md.

Refs #242: privileges for internal agents are a separate feature; this external operator surface does not widen internal defaults.