feat(atlas): Phase B tail — live namespaces + trim inline fallback

NamespaceSummary (test-first) renders the substrate ns line from live cluster
namespaces (system-filtered, sorted, capped). Handler now overlays both node
specs and the ns line via a single cached liveOverlay(). Removed the inline
SUBSTRATE/STAGES/NS data — /api/atlas.json is the single source; the page shows
an error banner on fetch failure instead of stale data.

GPU model naming intentionally NOT done: koala's node has no GPU product label
(only nvidia.com/gpu count), so "1× GPU" is the API's truth — naming the RTX 5070
would need GPU-feature-discovery, out of scope.

Verified: build/vet/lint(0)/test green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/cluster/client.go

@@ -21,6 +21,9 @@ const (
 // Nodes returns the raw /api/v1/nodes JSON from the in-cluster API server.
 func Nodes() ([]byte, error) { return get("/api/v1/nodes") }
 
+// Namespaces returns the raw /api/v1/namespaces JSON from the in-cluster API server.
+func Namespaces() ([]byte, error) { return get("/api/v1/namespaces") }
+
 func get(path string) ([]byte, error) {
 	host, port := os.Getenv("KUBERNETES_SERVICE_HOST"), os.Getenv("KUBERNETES_SERVICE_PORT")
 	if host == "" || port == "" {