feat(workflow): delay action with deferred job queue

[mpge]

Summary

Adds the `delay` workflow action to WordPress as the ninth entry in the catalog. Splits execution around a wait clause: actions before the delay run inline, remaining actions are persisted as an `escalated_deferred_workflow_jobs` row with `run_at = now + N seconds`. A WP-Cron handler running on the shared `escalated_every_minute` interval picks up due pending jobs, re-invokes the executor with the stored JSON, and flips the row to `done` / `failed` (retained for audit).

What's added

• `Models\DeferredWorkflowJob` — `find/create/update/pending` helpers matching the existing Reply/Tag/Ticket model patterns
• `Cron\Deferred_Workflow_Jobs_Check` registered on the new `escalated_run_due_deferred_workflow_jobs` hook
• Activator — `CREATE TABLE escalated_deferred_workflow_jobs (id, ticket_id, remaining_actions LONGTEXT, run_at DATETIME, status VARCHAR(16), last_error TEXT, ...)` with composite `(status, run_at)` index for efficient polling + `wp_schedule_event` for the minute-cadence sweep
• Bootstrap — registers the new cron handler alongside `Snooze_Check`
• Executor — detects `"delay"` before dispatching, serializes the remainder via `wp_json_encode`, persists the job, and short-circuits the rest of the run. Includes public `run_due_deferred_jobs()` method so the cron handler can invoke it
• 5 new PHPUnit cases: delay pauses + persists correctly, invalid value skips cleanly without persisting or running the tail, resumer processes a due row end-to-end and flips to done, resumer marks failed when ticket has been deleted, resumer correctly skips rows not yet due

Upgrade note

Existing installs need to reactivate the plugin to pick up the new table (via `deactivate → activate`). New installs get it automatically. This is a pre-existing infrastructure gap unrelated to this PR — a proper `plugins_loaded` version-check migration path is a separate future concern.

Base / Stacking

Stacked on `feat/workflow-executor` (PR #28). CI won't exercise this branch until that base merges and this rebases onto `main`.

Test plan

• `php -l` passes on all modified + new files
• CI runs `vendor/bin/phpunit` — 5 new tests in `Test_Workflow_Executor_Service` should pass
• Reviewer: confirm the activator upgrade path matches what the team expects (existing installs need reactivation)
• Reviewer: sanity-check the `run_at` UTC semantics (`gmdate('Y-m-d H:i:s', time() + $seconds)` writes UTC; `current_time('mysql')` in `pending()` reads the WP timezone setting — this should match because `pending()` uses `$wpdb->prepare` and relies on the DB comparing strings lexicographically). If the host app is configured with a non-UTC timezone, we'd want to normalize. Flagging for review.