Add conversation session abilities

[chubes4]

Summary

• Add generic conversation session abilities for listing, reading, creating, renaming, and deleting host-backed transcript sessions.
• Add a wp_agent_conversation_store discovery seam and list_sessions() contract to support frontend session lists without product tables.
• Cover the contract with a pure-PHP smoke test included in composer test.

Fixes #162.

Testing

• php -l agents-api.php && php -l src/Transcripts/class-wp-agent-conversation-store.php && php -l src/Transcripts/class-wp-agent-conversation-sessions.php && php -l src/Transcripts/register-agents-conversation-session-abilities.php && php -l tests/agents-conversation-session-abilities-smoke.php
• php tests/agents-conversation-session-abilities-smoke.php
• composer test
• homeboy lint --path /Users/chubes/Developer/agents-api@conversation-session-abilities --extension wordpress --errors-only
• homeboy test --path /Users/chubes/Developer/agents-api@conversation-session-abilities --extension wordpress

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the generic abilities, store discovery seam, smoke coverage, and cleanup changes; Chris remains responsible for review and testing.