Add pending action observer contract

[chubes4]

Closes #128

Summary

• Adds WP_Agent_Pending_Action_Observer for pending action lifecycle events.
• Loads the new interface from the package bootstrap and lists it in the public surface.
• Documents that stores own observer registration/invocation and should isolate observer failures from durable approval transitions.

Verification

• php tests/pending-action-store-contract-smoke.php
• php tests/bootstrap-smoke.php
• composer test
• homeboy lint --path /Users/chubes/Developer/agents-api@feat-pending-action-observer-contract --extension wordpress --changed-since origin/main
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the small observer interface, docs, and smoke-test updates; Chris remains responsible for review and merge.