P1: Add a versioned external backend adapter contract for OpenCode, ChatGPT Web and Orca

[Delqhi]

Problem

The control plane needs replaceable workers, but external sessions currently expose different identifiers, progress signals, cancellation behavior and result formats.

Required implementation

Define an adapter interface with:

• dispatch/resume/cancel/probe operations;
• durable external session and terminal identifiers;
• heartbeat and semantic-progress observations;
• structured callbacks/results and checkpoint support;
• process-group or remote-session ownership;
• retry/error classification and capability declarations.

Provide a fake adapter and reference integration contracts for OpenCode, ChatGPT Web/SIN-Chrome and Orca terminals.

Acceptance criteria

• Supervisor tests run entirely against the fake adapter.
• Backend replacement does not change goal/attempt semantics.
• Stale external sessions and duplicate callbacks are handled idempotently.
• Adapter outputs are schema-versioned and secret-redacted.