Clarify conversation loop event observers

[chubes4]

Summary

• Keep the caller-owned on_event callable and emit the same lifecycle events through a documented agents_api_loop_event WordPress action for independent observers.
• Document event payloads as read-only observational snapshots and make both callable/action observer failures non-fatal to loop execution.
• Cover action observer delivery and action observer failure behavior in the conversation loop event smoke test.

Closes #75.

Tests

• php -l src/Runtime/AgentConversationLoop.php
• php -l tests/conversation-loop-events-smoke.php
• php tests/conversation-loop-events-smoke.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Inspecting AgentConversationLoop event behavior, drafting the substrate-level observer contract, implementing the change, and running validation. Chris remains responsible for review and merge decisions.