Replace legacy pending-action resolved hook payload with canonical Agents API action

[chubes4]

Problem

WordPressActionDispatchObserver receives canonical Agents API pending-action objects, but on_resolved() still builds and emits a legacy Data Machine payload shape.

Code references on origin/main (008878b6):

• inc/Engine/AI/Actions/WordPressActionDispatchObserver.php:25 — builds $payload = $this->legacy_payload( $action ).
• inc/Engine/AI/Actions/WordPressActionDispatchObserver.php:45 — legacy_payload() maps Agents API action data into old DM field names.

Related closed issue: #1928.

Why this matters

The observer interface is already object-shaped. Keeping the old hook argument contract alive duplicates pending-action shapes and makes extension consumers pick between old arrays and canonical objects.

Acceptance criteria

• Audit current consumers of datamachine_pending_action_resolved.
• Decide the 1.0 hook signature: canonical WP_Agent_Pending_Action / decision object, or legacy array retained intentionally.
• Remove legacy_payload() if no longer needed, or move legacy details under metadata.datamachine with clear docs.
• Update docs/tests for the final hook signature.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: 1.0 technical-debt audit and issue drafting; Chris requested tracking issues for the findings.