refactor(permissions): move ghost-perm detection server-side

Add GET /api/permissions/stale endpoint that returns agent names with
explicit capability/tool-group JSON entries but no live container AND
no kept-state tombstone. Ghost detection is now entirely server-side —
one authoritative call, no client-side roster cache, no staleness window.

The previous client-side approach in core.js made three parallel API
calls (GET /api/capabilities, GET /api/tool-groups, GET /api/state) and
filtered the result against a module-level `liveContainerNames` Set
populated only on cold load and form submits. Any container lifecycle
event (spawn, destroy) while core.html was open left `liveContainerNames`
stale, risking a false-positive ghost entry for a live container.

Changes:
- permissions.rs: add `get_stale_permissions` handler + `StalePermsResponse`
  struct. Computes live roster (containers_snapshot), tombstone set
  (Coordinator::kept_state_names), explicit perm names (capabilities::read
  + tool_groups::read), then returns the difference sorted.
- dashboard.rs: register GET /api/permissions/stale.
- core.js: replace the three-call client-side logic in
  `fetchAndRenderStalePerms` with a single fetch to /api/permissions/stale.
  Remove `liveContainerNames` state + its syncFromSnapshot population.

frontend/packages/dashboard/src/core.js

@@ -20,15 +20,11 @@ let metaInputsState = [];
 let metaUpdateRunning = false;
 let tombstonesState = [];
 let rebuildQueueState = [];
-// Live container names — used by the stale-perms ghost filter.
-let liveContainerNames = new Set();
-
 function syncFromSnapshot(s) {
   metaInputsState = (s.meta_inputs || []).slice();
   metaUpdateRunning = !!s.meta_update_running;
   tombstonesState = (s.tombstones || []).slice();
   rebuildQueueState = (s.rebuild_queue || []).slice();
-  liveContainerNames = new Set((s.containers || []).map((c) => c.name));
 }
 
 // ─── meta inputs ──────────────────────────────────────────────────────────
@@ -452,8 +448,11 @@ function renderTombstones(s) {
 
 // ─── stale permission entries (K3PT ST4T3 pane sub-section) ──────────────
 // Agents with explicit capability / tool-group JSON entries but no live
-// container (e.g. renamed agents like the old "root" manager name). Lazy-
-// loaded on first "kept" tab activation; refreshed when perm data changes.
+// container AND no kept-state tombstone (e.g. renamed agents like the old
+// "root" manager name). Ghost detection is server-side via
+// GET /api/permissions/stale so the client doesn't need to maintain a
+// container-roster cache or perform set arithmetic. Lazy-loaded on first
+// "kept" tab activation; refreshed when perm data changes via SSE.
 let stalePermsLoaded = false;
 
 function renderStalePerms(root, ghosts) {
@@ -462,7 +461,7 @@ function renderStalePerms(root, ghosts) {
   root.append(el('p', { class: 'tombstones-stale-heading' }, 'stale permission entries'));
   root.append(el('p', { class: 'meta' },
     'agents with explicit capability or tool-group entries but no live container '
-    + '(typically renamed or manually-deleted agents whose JSON entries persisted).'));
+    + 'or kept state (typically renamed or manually-deleted agents whose JSON entries persisted).'));
   const errP = el('p', { class: 'tombstones-stale-err', hidden: true });
   const ul = el('ul', { class: 'tombstones-stale-list' });
   for (const name of ghosts) {
@@ -500,25 +499,21 @@ function renderStalePerms(root, ghosts) {
   root.append(ul, errP);
 }
 
+// Ghost detection is entirely server-side: GET /api/permissions/stale
+// returns the computed list of agent names that have explicit JSON entries
+// but are absent from both the live roster and the kept-state tombstones.
+// One call, no client-side roster cache, always authoritative.
 async function fetchAndRenderStalePerms() {
   const root = $('tombstones-stale-perms');
   if (!root) return;
   try {
-    const [capsResp, tgResp] = await Promise.all([
-      fetch('/api/capabilities'),
-      fetch('/api/tool-groups'),
-    ]);
-    const capsData = capsResp.ok ? await capsResp.json() : { assignments: {} };
-    const tgData = tgResp.ok ? await tgResp.json() : { assignments: {} };
-    const explicit = new Set([
-      ...Object.keys(capsData.assignments || {}),
-      ...Object.keys(tgData.assignments || {}),
-    ]);
-    const ghosts = [...explicit].filter((n) => !liveContainerNames.has(n)).sort();
-    renderStalePerms(root, ghosts);
+    const resp = await fetch('/api/permissions/stale');
+    if (!resp.ok) throw new Error('http ' + resp.status);
+    const data = await resp.json();
+    renderStalePerms(root, data.stale || []);
   } catch (err) {
     root.replaceChildren();
-    root.append(el('p', { class: 'meta' }, 'failed to load perm data: ' + err));
+    root.append(el('p', { class: 'meta' }, 'failed to load stale perm data: ' + err));
   }
   stalePermsLoaded = true;
 }