Document external chat run bridges

[chubes4]

Summary

• Document existing chat run-control filters as a generic external durable-run adapter contract.
• Add orchestration metadata conventions for provider, run_id, and event_cursor.
• Add a provider-agnostic fixture mapping external durable status/events into Agents API outputs for headless orchestration.

Verification

• php tests/chat-run-control-smoke.php
• php tests/external-chat-run-bridge-fixture-smoke.php
• composer validate --no-check-publish
• composer smoke
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Implementing adapter fixture coverage, updating provider-agnostic docs, and drafting this PR description.