Expose runtime tool context schema

[chubes4]

Summary

• Adds explicit agents/chat client context schema fields for trusted runtime-local tool declarations and callbacks.
• Keeps runtime tool context auditable instead of relying on undeclared ambient payload keys.

Testing

• php -l src/Channels/register-agents-chat-ability.php
• php tests/agents-chat-ability-smoke.php
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Diagnosing the Studio Web browser runtime tool contract failure, drafting the schema/test change, and running focused verification.