runs: add non-destructive flag + operator Cancel button

Non-destructive pre-declares "don't touch the disks" on Start: the
Storage stage skips wipe-probe, badblocks -w, and write-mode fio,
and reports a read-only summary. Runs a new non_destructive column;
threaded through Claim → agent tests.Deps → Storage stage.

Cancel halts an in-flight run. The orchestrator transitions to a
new StateCancelled via TriggerOperatorCancelled (valid from any
active state); the agent's next heartbeat returns cmd=cancel_stage,
which fires a stored CancelFunc on the per-stage context. Stage
subprocesses spawned with exec.CommandContext die with the context,
the agent posts a cancelled outcome, then powers the host off.

Destructive stages mid-run may leave the host in an intermediate
state — the UI confirm dialog warns the operator; recovery is
manual for now.

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

agent/tests/stage.go

@@ -46,6 +46,7 @@ type Deps struct {
 	Error          func(string)
 	Sensor         func(ctx context.Context, samples []Sample) error
 	OverrideWipe   bool
+	NonDestructive bool           // skip wipe-probe + writes in Storage
 	ExpectedDisks  []ExpectedDisk // serials + sizes from host.expected_spec
 	StageTimeout   time.Duration
 }