Fix principal-owned session reads

[chubes4]

Summary

• Add an optional owner-aware single-session read contract for principal-owned transcript stores.
• Use the owner-aware read path when loading one conversation session, so stores with hashed owner keys can verify ownership internally.
• Extend the conversation session smoke test to cover matching-owner loads and cross-owner denial.

Testing

• php tests/agents-conversation-session-abilities-smoke.php
• homeboy lint --changed-since origin/main
• homeboy test --changed-since origin/main

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Diagnosed the anonymous session load ownership gap, drafted the contract/test change, and ran scoped verification; Chris remains responsible for review and merge.