slice 10B: signed-op job completion (DELETE clear-job) (hub v0.10.0)

Add DELETE /hosts/{id}/jobs/{job_id} (per-host self-scoped, idempotent) so the
agent clears a job after executing or terminally rejecting it. The hub stores
the operator-signed blobs opaquely (no signing key — cannot forge or open);
the agent verifies + executes. Doc 03 §4/§6/§9 updated (operator-signed path
live; 8C wipe completes; 10B done).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hub/internal/store/store.go

@@ -1517,6 +1517,14 @@ func (s *Store) CountSignedJobs(hostID string) (int, error) {
 	return n, err
 }
 
+// DeleteSignedJob removes a processed job from a host's queue (slice 10B completion). The agent
+// calls it after executing OR terminally rejecting a job. Idempotent (deleting an absent job is a
+// no-op, returns nil) — a retried completion must not error.
+func (s *Store) DeleteSignedJob(hostID, jobID string) error {
+	_, err := s.db.Exec(`DELETE FROM signed_jobs WHERE host_id = ? AND job_id = ?`, hostID, jobID)
+	return err
+}
+
 // SaveHostReport inserts a host_reports row and bumps the host's reality columns
 // (agent_version/last_report_at/updated_at) — never the inert intent columns.
 func (s *Store) SaveHostReport(hostID, customerID string, reportJSON []byte, d HostReportDenorm) error {