Add canonical chat run control

[chubes4]

Summary

• Adds canonical chat run IDs to agents/chat, including generated run IDs passed to runtimes and returned to clients.
• Registers generic agents/get-chat-run, agents/cancel-chat-run, and agents/queue-chat-message abilities backed by runtime-provided handlers.
• Documents the runtime-agnostic run-control contract and adds smoke coverage for status, cancellation, queueing, and interrupt mapping.

Fixes #236.

Testing

• php tests/chat-run-control-smoke.php
• php tests/frontend-chat-rest-smoke.php
• php tests/conversation-loop-interrupt-source-smoke.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted and implemented the generic run-control contract, tests, and docs for Chris to review and validate.