Add frontend chat REST adapter

[chubes4]

Summary

• Adds a generic POST /agents-api/v1/chat REST adapter that dispatches frontend chat turns through the canonical agents/chat ability.
• Composes permission through the existing chat permission filter plus current-principal agent access checks, with host-level REST filters for policy and input shaping.
• Adds pure-PHP smoke coverage for route registration, dispatch payloads, access denial, filterability, and invalid input handling.

Testing

• php -l agents-api.php && php -l src/Channels/register-frontend-chat-rest-route.php && php -l tests/agents-api-smoke-helpers.php && php -l tests/frontend-chat-rest-smoke.php
• php tests/frontend-chat-rest-smoke.php
• composer test
• homeboy lint --path /Users/chubes/Developer/agents-api@frontend-chat-rest-adapter --extension wordpress --errors-only
• homeboy test --path /Users/chubes/Developer/agents-api@frontend-chat-rest-adapter --extension wordpress

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the generic REST adapter, smoke coverage, simplify cleanup, and verification commands for Chris to review.