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>

server/pkg/db/generated/issue.sql.go

@@ -231,3 +231,42 @@ func (q *Queries) UpdateIssue(ctx context.Context, arg UpdateIssueParams) (Issue
 	)
 	return i, err
 }
+
+const updateIssueStatus = `-- name: UpdateIssueStatus :one
+UPDATE issue SET
+    status = $2,
+    updated_at = now()
+WHERE id = $1
+RETURNING id, workspace_id, title, description, status, priority, assignee_type, assignee_id, creator_type, creator_id, parent_issue_id, acceptance_criteria, context_refs, repository, position, due_date, created_at, updated_at
+`
+
+type UpdateIssueStatusParams struct {
+	ID     pgtype.UUID `json:"id"`
+	Status string      `json:"status"`
+}
+
+func (q *Queries) UpdateIssueStatus(ctx context.Context, arg UpdateIssueStatusParams) (Issue, error) {
+	row := q.db.QueryRow(ctx, updateIssueStatus, arg.ID, arg.Status)
+	var i Issue
+	err := row.Scan(
+		&i.ID,
+		&i.WorkspaceID,
+		&i.Title,
+		&i.Description,
+		&i.Status,
+		&i.Priority,
+		&i.AssigneeType,
+		&i.AssigneeID,
+		&i.CreatorType,
+		&i.CreatorID,
+		&i.ParentIssueID,
+		&i.AcceptanceCriteria,
+		&i.ContextRefs,
+		&i.Repository,
+		&i.Position,
+		&i.DueDate,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+	)
+	return i, err
+}