Add principal-owned conversation sessions

[chubes4]

Summary

• Add a canonical conversation-owner shape to execution principals.
• Introduce an optional principal-aware conversation store contract for non-user session owners.
• Keep legacy user-owned conversation stores working for user principals.

Testing

• php -l agents-api.php src/Runtime/class-wp-agent-execution-principal.php src/Transcripts/class-wp-agent-principal-conversation-store.php src/Transcripts/register-agents-conversation-session-abilities.php tests/agents-conversation-session-abilities-smoke.php tests/execution-principal-smoke.php
• php tests/execution-principal-smoke.php
• php tests/agents-conversation-session-abilities-smoke.php
• composer test
• homeboy lint --changed-only --force-hot
• homeboy test --changed-only --force-hot

Related

• Fixes Support principal-owned conversation sessions #174.
• Builds on Support non-user audience principals for public and proxy-authenticated agents #172 / Add audience principals for agent access #173.
• Downstream: Separate chat transcript ownership from runtime execution user Extra-Chill/data-machine#2023 and Persist logged-out chat sessions with a secure browser principal Extra-Chill/frontend-agent-chat#19

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting the implementation, tests, and PR description; Chris remains responsible for review and validation.