Event triggers: WP_Agent_Event_Trigger primitive symmetric to WP_Agent_Routine

[lezama]

Problem

WP_Agent_Routine + WP_Agent_Routine_Registry give us a first-class primitive for scheduled agent runs (cron interval, persistent session, agent slug, prompt). There is no symmetric primitive for hook-triggered runs: "WordPress hook fires → resolve prompt → dispatch agent". Every consumer that wants "react to a do_action and run an agent" re-implements the dispatch wiring (hook attachment, placeholder substitution, async hop).

Proposal

Mirror the Routines/ directory with a new Triggers/ directory.

Value object

WP_Agent_Event_Trigger with:

• id, agent_slug, enabled
• hook_name (e.g. transition_post_status, wp_login, woocommerce_order_status_changed)
• args_shape[] — declared payload keys exposed to placeholders
• placeholders[] — flat dotted keys (post.title, user.email, order.total) with extractor callbacks
• conditions[] — pre-dispatch filters (post_type, status, role, …)
• prompt_template — string with {{placeholder}} substitution

Registry + helper

• WP_Agent_Event_Trigger_Registry — register/unregister/list, mirrors WP_Agent_Routine_Registry.
• register_event_trigger_handler( WP_Agent_Event_Trigger \$trigger ) — attaches add_action( \$trigger->hook_name(), ... ) at a late priority, evaluates conditions, resolves placeholders, dispatches via wp_schedule_single_event so the user-facing request never blocks.

Boundary

Substrate owns the value object, registry, and dispatch wiring helper only. Concrete catalogs of triggers (WordPress core hooks, WooCommerce hooks, form-plugin hooks) stay consumer-owned, mirroring how workflow definitions are split today.

Acceptance

• src/Triggers/class-wp-agent-event-trigger.php (value object)
• src/Triggers/class-wp-agent-event-trigger-registry.php
• src/Triggers/register-event-trigger-handler.php (the wiring helper)
• Smoke: registration round-trip, placeholder resolution, condition evaluation, async dispatch path (assert wp_schedule_single_event is called)