refactor(routines): WP_Agent_Routine_Backend contract; Action Scheduler becomes the default backend

[chubes4]

Closes #544.

What

The routine registry now depends on a contract instead of naming Action Scheduler:

• interface WP_Agent_Routine_Backend (src/Routines/interface-wp-agent-routine-backend.php) carries the minimal surface the registry consumes: is_available(), register(), unregister(), pause(), resume(), run_now(), is_paused(), current_generation(), pending_by_routine(): array<string, list<int>>, and cancel( int $handle ). Handles are opaque ints; the registry never interprets them.
• WP_Agent_Routine_Action_Scheduler_Bridge is now an instance class (singleton via ::instance()) implementing the contract. Generation fencing, the option helpers (generation_option_name, action_generation*), and the fence installers (register_generation_fence, stamp_stored_action, fence_before_execute) stay on the bridge as AS-specific plumbing, not on the interface.
• WP_Agent_Routine_Registry::backend(): ?WP_Agent_Routine_Backend resolves once per request via apply_filters( 'wp_agent_routine_backend', $default ), where $default is the AS bridge instance when WP_Agent_Routine_Action_Scheduler_Bridge::instance()->is_available() is true, else null. A filter return that is not a WP_Agent_Routine_Backend falls back to the default. WP_Agent_Routine_Registry::reset_backend() supports tests. Every direct bridge call in the registry is gone; reconcile() works purely off the interface and returns the _scheduler error when no backend resolves.
• register-routine-bridge-sync.php is backend-agnostic: the five lifecycle hooks call WP_Agent_Routine_Registry::backend()?->… and no-op when null. We kept the filename (avoids churn in agents-api.php ordering); its docblock now describes backend sync, not AS sync.

Deprecation shim policy

The issue suggested keeping the static facade as deprecation shims, but PHP cannot carry a static and an instance method of the same name — and the instance forms are required by the interface. The static facade for the interface method names (register, unregister, pause, resume, run_now, is_available, is_paused, current_generation) is therefore removed outright; per the issue's own grep, only agents-api itself called them, and all in-tree call sites are migrated in this PR. The two non-colliding statics remain as thin @deprecated 0.11.0 shims: pending_routine_actions() → pending_by_routine(), cancel_action_by_id() → cancel(). agents-api.php now explicitly requires the interface before the implementing class (first-boot loading, matching the Workflows branch-executor pattern).

Vendor hook removal (layer purity)

src/Routines/register-action-scheduler-listener.php no longer adds/removes the openclawp_chat_ability_permission filter — a vendor/product hook name in the generic substrate. The listener already grants scheduled-dispatch permission through the generic agents_chat_permission filter; the product that needs the other hook should subscribe to agents_chat_permission on its side (or register its own filter from its own plugin). A grep of src/ confirms no vendor names remain in the Routines module.

Tests

New tests/routines-backend-contract-smoke.php (registered in composer.json scripts.smoke right after the durability smoke) runs with no as_* functions defined and proves: a fake in-memory backend installed via the wp_agent_routine_backend filter receives register with the exact routine instance; pause/resume/run_now/unregister route through; reconcile() uses pending_by_routine()/cancel()/register() and returns the enqueued/removed/unchanged shape (including dry-run and paused-routine cases); a garbage filter return falls back to the null default and reconcile() reports _scheduler while lifecycle verbs stay inert-safe.

All three routine smokes are green: routine-smoke (36), routines-durability-smoke (56 — fencing/reconcile behavior unchanged), routines-backend-contract-smoke (35). Full composer smoke suite passes; composer phpstan reports no errors; phpcbf (Generic.Formatting.MultipleStatementAlignment) clean on every changed file.

Docs: docs/channels-workflows-operations.md gains a "Routine backends" subsection (the interface, the filter, AS as the default backend when present, routines registered but inert with no backend).

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