docs: document the generic harness integration loop

[joryirving]

Parent: #392

Document how any harness integrates with Dispatch without custom runtime scripts.

Docs should cover:

• ask for next task
• exit on idle
• execute exactly one task
• report result
• stop

Examples:

• generic curl or CLI flow
• OpenClaw scheduler flow
• OpenCode/manual flow
• Codex/Claude Code style one-shot flow

Acceptance:

• docs are harness-agnostic first
• harness examples are small and optional
• no second runtime repo required
• worker and groomer loops are documented separately