fix(triggers): durable completion-hook delivery — retry, dead-letter, redeliver (closes #97)

[umi-appcoder]

What

Makes completion-hook delivery durable — bounded retries, a dead-letter record on exhaustion, and a redeliver endpoint. Closes #97 — PR 5 (finale) of the reliability-hardening series. Builds on the merged #96 reconcile loop.

Why

_fire_completion_hook was fire-and-forget — a single network blip dropped the callback, which is silent data loss for webhook/cron triggers whose whole purpose is the response_url POST.

Backend (server.py)

• _deliver_hook — bounded exponential-backoff retry (KC_HOOK_MAX_ATTEMPTS, default 4); a permanent 4xx (except 429) isn't retried. Records delivery state on the task meta:
  • success → hook_delivery = {state:'delivered', attempts, status, delivered_at}
  • exhaustion → hook_delivery = {state:'failed', attempts, last_error, last_attempt_at} — the dead-letter.
• _build_hook_request — extracted so fire + redeliver share payload/HMAC.
• redeliver_hook + POST /api/claude/tasks/{id}/redeliver-hook (auth + readonly gated) to re-attempt a failed delivery.
• At-most-once on success preserved (hook_fired_at marks the initial fire under lock); retries run in the daemon thread, off the request path.

SPA

TaskDetail Info tab surfaces hook_delivery state (delivered / failed + attempts + error, with the redeliver endpoint noted); TaskDetail type gains HookDelivery.

Tests

tests/completion_hook_delivery_test.py (7): success→delivered, retry→dead-letter, permanent-4xx no-retry, 429 retried, redeliver missing/no-url/runs-delivery. Python suite 473, SPA 120, OK.

Closes #97

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