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>

internal/store/store_test.go

@@ -36,7 +36,7 @@ func seedRun(t *testing.T, runs *store.Runs) (int64, int64) {
 	if err != nil {
 		t.Fatalf("create host: %v", err)
 	}
-	runID, err := runs.Create(context.Background(), hostID, "deadbeef")
+	runID, err := runs.Create(context.Background(), hostID, "deadbeef", false)
 	if err != nil {
 		t.Fatalf("create run: %v", err)
 	}