fix(#3020): K3PT ST4T3 says what it shows, and stops reading the job graph

`build_tombstone_views` folded `transient_snapshot`'s keys into its `live`
set, so an agent with in-flight transient work was treated as not-a-tombstone.
Since #3010 that set is derived from the running job graph, which made a
page about on-disk state a function of the scheduler.

Dropping the filter exposes what was always true underneath: nothing records
a destroy. Every definition-side artifact — state subvolume, proposed +
applied repos, `deployed/0`, meta registration, topology entry — is written by
`Provision` before the container exists and survives `lifecycle::destroy`. So
a mid-spawn agent is byte-identical on disk to a tombstone.

Per mara on #3020: remove the filter, warn on the page, keep the issue open
for the swarm-controller / snapshot-storage rework where the problem shape
changes anyway.

- dashboard/tombstones.rs: drop the param + the chain; document the real
  semantics
- core.js: amber caveat banner above the rows; row badge `destroyed` ->
  `offline`, which is what an absent container actually proves
- dashboard.css: `.tombstone-warn`, modelled on `.port-conflict` but amber and
  without the pulse — a permanent banner that pulses trains you to ignore it
- docs/web-ui/dashboard.md: the pane was described as "destroyed-but-state-kept
  agents", now the exact wrong claim

frontend/packages/dashboard/src/core.js

@@ -46,13 +46,27 @@ function renderTombstones(s) {
     if (d === 1) return '1 day ago';
     return d + ' days ago';
   };
+  // Only shown alongside actual rows — a caveat over an empty list is noise.
+  // Wording is deliberately about what the list *is* rather than what it
+  // isn't: nothing records a destroy, so "container absent" is the only thing
+  // the backend can actually tell.
+  const warn = el('p', { class: 'tombstone-warn' });
+  warn.append(
+    el('strong', {}, 'shows every agent whose container is absent'),
+    ' — not only destroyed ones. An agent part-way through being spawned ' +
+    'looks identical here, because nothing records a destroy. Check it is ' +
+    'really gone before you PURG3.',
+  );
+  root.append(warn);
   const ul = el('ul', { class: 'containers' });
   for (const t of s.tombstones) {
     const li = el('li', { class: 'container-row tombstone' });
     const head = el('div', { class: 'head' });
     head.append(
       el('span', { class: 'name' }, t.name),
-      el('span', { class: 'badge badge-muted' }, 'destroyed'),
+      // Was `destroyed`, which the backend cannot actually know — see the
+      // caveat above. `offline` is what the absence of a container proves.
+      el('span', { class: 'badge badge-muted' }, 'offline'),
     );
     if (t.has_creds) head.append(el('span', { class: 'badge badge-muted' }, 'creds kept'));
     head.append(el('span', { class: 'meta' },

frontend/packages/dashboard/src/dashboard.css

@@ -444,6 +444,21 @@ hive-agent-menu {
 }
 .container-row.tombstone .name { color: var(--muted); }
 
+/* K3PT ST4T3 caveat. Amber, not red like `.port-conflict`, and no pulse: this
+   is a standing "read the list this way" note, not an incident — a pulsing
+   banner that is always present just teaches you to stop seeing it.
+   Nothing records a destroy, so this list is every agent whose container is
+   absent — which includes one being spawned right now. */
+.tombstone-warn {
+  background: color-mix(in srgb, var(--amber) 8%, transparent);
+  border: 1px solid var(--amber);
+  color: var(--amber);
+  padding: 0.5em 0.8em;
+  margin-bottom: 0.6em;
+  border-radius: 4px;
+}
+.tombstone-warn strong { color: var(--amber); }
+
 /* Notification controls — sit between the banner and the containers. */
 .port-conflict {
   background: color-mix(in srgb, var(--red) 8%, transparent);