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.

hive-c0re/src/dashboard.rs

@@ -115,6 +115,10 @@ pub async fn serve(port: u16, coord: Arc<Coordinator>) -> Result<()> {
             post(permissions::post_capabilities),
         )
         .route("/api/permissions", post(permissions::post_permissions))
+        .route(
+            "/api/permissions/stale",
+            get(permissions::get_stale_permissions),
+        )
         .route(
             "/api/permissions/{agent}",
             axum::routing::delete(permissions::delete_agent_permissions),