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/db/migrations/0003_add_runs_non_destructive.sql

@@ -0,0 +1 @@
+ALTER TABLE runs ADD COLUMN non_destructive INTEGER NOT NULL DEFAULT 0;