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/api/ui_handlers.go

@@ -660,7 +660,11 @@ func (u *UI) CancelRun(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
-	if latest == nil || latest.State.IsTerminal() {
+	// FailedHolding is "terminal" for the pipeline but the host is still
+	// alive and holding at an SSH prompt; the operator can walk away from
+	// the hold via Cancel (which reboots to local disk). Every other
+	// terminal state is truly done — nothing to cancel.
+	if latest == nil || (latest.State.IsTerminal() && latest.State != model.StateFailedHolding) {
 		http.Error(w, "no active run to cancel", http.StatusConflict)
 		return
 	}