feat(server): add task service layer and daemon REST protocol
- Extract TaskService (server/internal/service/task.go) for task lifecycle:
enqueue with context snapshot, claim, start, complete, fail, progress
- Add daemon protocol endpoints under /api/daemon/:
register, heartbeat, claim task, start/progress/complete/fail task
- Task ↔ Issue status sync: running→in_progress, completed→in_review, failed→blocked
- Agent status auto-management: reconcile idle/working based on running tasks
- Enforce max_concurrent_tasks on task claiming (FOR UPDATE SKIP LOCKED)
- Add UpdateIssueStatus query (fixes bug where UpdateIssue nulls assignee)
- Extract shared pgx utils to server/internal/util/ to avoid circular imports
- Migration 003: add context JSONB to agent_task_queue, daemon unique constraint
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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