chore(jobs): align log keys + add prune no-op test + down-migration comment

Cross-cutting review polish:
- worker.go: id/type → job_id/job_type; one stray attempts → attempt for
  consistency with the other 4 sites in the same file.
- runners/workspace_fetch.go: ws_id → workspace_id (matches the dispatched
  notify.Message metadata key).
- runners/worktree_prune.go: id → worktree_id (same reason).
- runners/worktree_prune_test.go: add TestWorktreePrune_NoRowsIsNoOp for
  symmetry with the no-op tests in the other 4 runner test files.
- migrations/0005_jobs.down.sql: comment that DROP TABLE jobs cascades to
  drop the three idx_jobs_* indexes automatically.

internal/jobs/runners/worktree_prune_test.go

@@ -155,6 +155,23 @@ func TestWorktreePrune_ExecutionFailureRollsBack(t *testing.T) {
 	assert.Equal(t, "rollback", repo.finalized[id])
 }
 
+func TestWorktreePrune_NoRowsIsNoOp(t *testing.T) {
+	t.Parallel()
+	repo := &fakeWorktreeRepo{} // both lists nil
+	gitr := &fakePruner{}
+	disp := &recordingDispatcher{}
+	cfg := runners.WorktreePruneConfig{IdleThreshold: 7 * 24 * time.Hour, WarningLead: 3 * 24 * time.Hour}
+
+	r := runners.NewWorktreePrune(repo, gitr, disp, cfg, discardLogger())
+	r.Run(context.Background())
+
+	assert.Empty(t, repo.markedWarning)
+	assert.Empty(t, repo.tryStartCalled)
+	assert.Empty(t, repo.finalized)
+	assert.Zero(t, gitr.calls)
+	assert.Empty(t, disp.msgs)
+}
+
 func TestWorktreePrune_CASLostSkipsSilently(t *testing.T) {
 	t.Parallel()
 	id := uuid.New()