feat: implement ControlTower TUI for cluster and host monitoring

Add complete TUI application for monitoring Kubernetes clusters and host
systems. Features include:

Core features:
- Collector framework with concurrent scheduling
- Host collectors: disk, memory, load, network
- Kubernetes collectors: pods, nodes, workloads, events with informers
- Issue deduplication, state management, and resolve-after logic
- Bubble Tea TUI with table view, details pane, and filtering
- JSON export functionality

UX improvements:
- Help overlay with keybindings
- Priority/category filters with visual indicators
- Direct priority jump (0/1/2/3)
- Bulk acknowledge (Shift+A)
- Clipboard copy (y)
- Theme toggle (T)
- Age format toggle (d)
- Wide title toggle (t)
- Vi-style navigation (j/k)
- Home/End jump (g/G)
- Rollup drill-down in details

Robustness:
- Grace period for unreachable clusters
- Rollups for high-volume issues
- Flap suppression
- RBAC error handling

Files: All core application code with tests for host collectors,
engine, store, model, and export packages.

internal/collectors/host/net_test.go

@@ -0,0 +1,28 @@
+package host
+
+import "testing"
+
+func TestHasDefaultRoute(t *testing.T) {
+	in := "Iface\tDestination\tGateway\tFlags\n" +
+		"eth0\t00000000\t0102A8C0\t0003\n"
+	if !hasDefaultRoute(in) {
+		t.Fatalf("expected default route")
+	}
+	in2 := "Iface Destination Gateway Flags\n" +
+		"eth0 0010A8C0 00000000 0001\n"
+	if hasDefaultRoute(in2) {
+		t.Fatalf("expected no default route")
+	}
+}
+
+func TestIsIfaceUp(t *testing.T) {
+	if !isIfaceUp("up\n") {
+		t.Fatalf("expected true")
+	}
+	if !isIfaceUp("unknown") {
+		t.Fatalf("expected true for unknown")
+	}
+	if isIfaceUp("down") {
+		t.Fatalf("expected false")
+	}
+}