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/orchestrator/statemachine.go

@@ -73,7 +73,7 @@ var table = map[Trigger]transition{
 	TriggerStageMismatch:     {from: stageExecutionStates(), to: model.StateFailedHolding},
 	TriggerAllStagesPassed:   {from: []model.RunState{model.StateReporting}, to: model.StateCompleted},
 	TriggerOperatorReleased:  {from: []model.RunState{model.StateFailedHolding}, to: model.StateReleased},
-	TriggerOperatorCancelled: {from: allActiveStates(), to: model.StateCancelled},
+	TriggerOperatorCancelled: {from: append(allActiveStates(), model.StateFailedHolding), to: model.StateCancelled},
 }
 
 // Next computes the target state for a trigger against the current state.