Add canonical session owner contract

[chubes4]

Summary

• add session_owner to the canonical agents/chat input schema
• allow conversation session abilities to use an explicit isolating session_owner when the access principal has no owner
• reject shared public audience owners and keep explicit user owners bound to the authenticated user principal
• add smoke coverage for schema exposure and explicit owner handling

Testing

• php tests/agents-chat-ability-smoke.php
• php tests/agents-conversation-session-abilities-smoke.php
• homeboy lint --path /Users/chubes/Developer/agents-api@fix-session-owner-chat-contract --extension wordpress

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the implementation and smoke coverage; Chris remains responsible for review and validation.