Dispatch pending action lifecycle observers

[chubes4]

Summary

• Adds a Data Machine pending-action observer registry backed by the upstream Agents API WP_Agent_Pending_Action_Observer contract.
• Dispatches stored, resolved, and expired lifecycle events from PendingActionStore after durable transitions complete.
• Registers a WordPress hook adapter while preserving the legacy datamachine_pending_action_resolved signature.

Closes #1928.

Testing

• php tests/pending-actions-agents-api-contract-smoke.php
• php tests/pending-action-resolver-contract-smoke.php
• homeboy lint --path /Users/chubes/Developer/data-machine@feat-pending-action-observers --extension wordpress --changed-since origin/main
• homeboy test --path /Users/chubes/Developer/data-machine@feat-pending-action-observers --extension wordpress --changed-since origin/main
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implementing the pending-action observer dispatch path, smoke coverage, docs updates, and local validation; Chris remains responsible for review and merge decisions.