Add agent access abilities

[chubes4]

Summary

• Adds a host-store discovery seam for WP_Agent_Access_Store via wp_agent_access_store.
• Adds current-principal access helpers plus agents/can-access-agent and agents/list-accessible-agents Abilities API registrations.
• Adds pure-PHP smoke coverage and includes it in composer test.

Fixes #160.

Testing

• php -l src/Auth/class-wp-agent-access.php
• php -l src/Auth/register-agent-access-abilities.php
• php -l tests/agents-access-ability-smoke.php
• php tests/agents-access-ability-smoke.php
• composer test
• homeboy lint --path /Users/chubes/Developer/agents-api@agent-access-abilities --extension wordpress --errors-only
• homeboy test --path /Users/chubes/Developer/agents-api@agent-access-abilities --extension wordpress

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the access helper/ability implementation and smoke tests; Chris remains responsible for review and merge decisions.