feat(cancel): allow cancel from FailedHolding, reboot to local disk

A held run sits indefinitely at an SSH prompt waiting for operator
investigation. Previously the only exits were Override (re-enter the
failed stage) or leaving the host on forever — Cancel rejected any
terminal state, including FailedHolding, and there was no button in
the UI anyway.

Add a dedicated exit path:
  - statemachine: TriggerOperatorCancelled now accepts FailedHolding
    as a valid source, transitioning to Cancelled like any other
    live state.
  - CancelRun handler: treats FailedHolding as cancellable even
    though IsTerminal reports true.
  - heartbeat: Cancelled runs fork on FailedStage. Set means the
    agent is parked in waitForOverride with no subprocess in
    flight, so cmd=reboot tells it to systemctl reboot; the host
    falls through iPXE's no-active-run script to the local disk.
    Empty FailedStage keeps the pre-existing cmd=cancel_stage path
    for mid-stage cancels (kill stage ctx, then power off).
  - UI: canCancel now returns true for FailedHolding, and the
    run-detail page renders a distinct "Cancel & reboot" button
    with a hold-specific confirm message so the action doesn't
    look identical to a mid-run cancel.

Tests cover the new statemachine transition, the heartbeat fork
(reboot vs cancel_stage), and keep the pre-existing mid-run cancel
behaviour locked in.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/web/templates/host_tile_templ.go

@@ -284,12 +284,20 @@ func canStartIfOnline(r *model.Run) bool {
 	return r.State.IsTerminal()
 }
 
-// canCancel is true for any non-terminal run — the Cancel button shows
-// whenever the pipeline is live (Queued through the stage states). The
-// handler refuses the action once the run enters a terminal state, so
-// the render decision just has to mirror that.
+// canCancel is true for any non-terminal run, plus FailedHolding —
+// a held run technically classifies as terminal for the pipeline but
+// the host is still live on the SSH hold prompt, and the operator
+// can walk away from it via Cancel (which reboots to local disk).
+// Every other terminal state is truly done, so no Cancel button.
+// The server-side CancelRun handler mirrors this predicate.
 func canCancel(r *model.Run) bool {
-	return r != nil && !r.State.IsTerminal()
+	if r == nil {
+		return false
+	}
+	if !r.State.IsTerminal() {
+		return true
+	}
+	return r.State == model.StateFailedHolding
 }
 
 func tileStatus(r *model.Run) string {