feat(studio): provide a trusted host bridge for the local MCP server

[7shep]

Problem

Studio has a typed client and MCP transport adapter, but browsers cannot connect directly to stdin/stdout. There is no maintained trusted host bridge that wires the real local MCP session into Studio.

Suggested level: Advanced
Primary area: Host integration and apps/studio

Acceptance criteria

• Document the host architecture and trust boundary before implementation.
• Expose only Studio-required MCP operations through a typed, validated interface.
• Keep the complete serialized session current across every mutation.
• Handle reconnect, timeout, cancellation, and structured errors.
• Restrict origins and reject malformed/unauthorized requests.
• Expose no model credentials, arbitrary commands, or raw filesystem access.
• Preserve the deterministic fixture client.
• Test happy path, invalid session, transport failure, retry, and stale response.
• Update exact run/setup documentation.

Repository workflow

Visible Studio changes must follow AGENTS.md and complete the Universal implementation review.