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.
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