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/k8s/issues_nodes.go

@@ -0,0 +1,79 @@
+package k8s
+
+import (
+	"fmt"
+
+	corev1 "k8s.io/api/core/v1"
+
+	"tower/internal/model"
+)
+
+// IssuesFromNodes applies the PLAN.md node rules.
+//
+// Pure rule function: does not talk to the API server.
+func IssuesFromNodes(nodes []*corev1.Node) []model.Issue {
+	out := make([]model.Issue, 0, 8)
+	for _, n := range nodes {
+		if n == nil {
+			continue
+		}
+
+		// Ready / NotReady
+		if cond := findNodeCondition(n, corev1.NodeReady); cond != nil {
+			if cond.Status != corev1.ConditionTrue {
+				out = append(out, model.Issue{
+					ID:       fmt.Sprintf("k8s:node:%s:NotReady", n.Name),
+					Category: model.CategoryKubernetes,
+					Priority: model.PriorityP0,
+					Title:    fmt.Sprintf("Node NotReady: %s", n.Name),
+					Details:  cond.Message,
+					Evidence: map[string]string{
+						"kind":      "Node",
+						"reason":    "NotReady",
+						"namespace": "",
+						"node":      n.Name,
+						"status":    string(cond.Status),
+					},
+					SuggestedFix: "kubectl describe node " + n.Name,
+				})
+			}
+		}
+
+		// Pressure conditions.
+		for _, ctype := range []corev1.NodeConditionType{corev1.NodeMemoryPressure, corev1.NodeDiskPressure, corev1.NodePIDPressure} {
+			if cond := findNodeCondition(n, ctype); cond != nil {
+				if cond.Status == corev1.ConditionTrue {
+					out = append(out, model.Issue{
+						ID:       fmt.Sprintf("k8s:node:%s:%s", n.Name, string(ctype)),
+						Category: model.CategoryKubernetes,
+						Priority: model.PriorityP1,
+						Title:    fmt.Sprintf("Node %s: %s", ctype, n.Name),
+						Details:  cond.Message,
+						Evidence: map[string]string{
+							"kind":      "Node",
+							"reason":    string(ctype),
+							"namespace": "",
+							"node":      n.Name,
+							"status":    string(cond.Status),
+						},
+						SuggestedFix: "kubectl describe node " + n.Name,
+					})
+				}
+			}
+		}
+	}
+	return out
+}
+
+func findNodeCondition(n *corev1.Node, t corev1.NodeConditionType) *corev1.NodeCondition {
+	if n == nil {
+		return nil
+	}
+	for i := range n.Status.Conditions {
+		c := &n.Status.Conditions[i]
+		if c.Type == t {
+			return c
+		}
+	}
+	return nil
+}