Provide default chat run control

[chubes4]

Summary

• Makes chat run control default in Agents API instead of a separate capability probe.
• Stores generic run status for agents/chat, supports best-effort cancellation, and accepts queued follow-up messages when no runtime-specific handler is registered.
• Lets runtimes override status/cancel/queue hooks only when they can provide stronger behavior, while clients can rely on the canonical abilities by default.

Verification

• git diff --check
• php tests/chat-run-control-smoke.php
• php -l src/Runtime/class-wp-agent-chat-run-control.php && php -l src/Channels/register-agents-chat-run-control-abilities.php && php -l src/Channels/register-agents-chat-ability.php && php -l src/Runtime/class-wp-agent-conversation-loop.php && php -l tests/chat-run-control-smoke.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Revised the run-control implementation from a capability-probe model to default generic support, updated docs/tests, and ran validation. Chris remains responsible for review and merge.