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/model/issue_test.go

@@ -0,0 +1,75 @@
+package model
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+	"time"
+)
+
+func TestSortIssuesDefault_PriorityThenRecency(t *testing.T) {
+	t0 := time.Date(2025, 12, 1, 10, 0, 0, 0, time.UTC)
+
+	issues := []Issue{
+		{ID: "b", Priority: PriorityP1, LastSeen: t0.Add(10 * time.Second)},
+		{ID: "a", Priority: PriorityP0, LastSeen: t0.Add(1 * time.Second)},
+		{ID: "c", Priority: PriorityP1, LastSeen: t0.Add(20 * time.Second)},
+		{ID: "d", Priority: PriorityP2, LastSeen: t0.Add(30 * time.Second)},
+	}
+
+	SortIssuesDefault(issues)
+	got := []string{issues[0].ID, issues[1].ID, issues[2].ID, issues[3].ID}
+	want := []string{"a", "c", "b", "d"} // P0 first; within P1 higher LastSeen first
+
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("order mismatch: got %v want %v", got, want)
+	}
+}
+
+func TestJSONRoundTrip_EnumsStable(t *testing.T) {
+	when := time.Date(2025, 12, 20, 12, 0, 0, 0, time.UTC)
+	in := Issue{
+		ID:           "host:disk:/home:usage",
+		Category:     CategoryStorage,
+		Priority:     PriorityP1,
+		Title:        "Disk nearly full",
+		Details:      "Usage above threshold",
+		Evidence:     map[string]string{"mount": "/home", "used_pct": "93"},
+		SuggestedFix: "du -sh * | sort -h",
+		State:        StateOpen,
+		FirstSeen:    when,
+		LastSeen:     when.Add(5 * time.Second),
+	}
+
+	b, err := json.Marshal(in)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+
+	var out Issue
+	if err := json.Unmarshal(b, &out); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+
+	// Compare fields we care about; time.Time compares directly.
+	if in.ID != out.ID || in.Category != out.Category || in.Priority != out.Priority || in.State != out.State {
+		t.Fatalf("basic fields mismatch after round-trip: in=%+v out=%+v", in, out)
+	}
+	if in.Title != out.Title || in.Details != out.Details || in.SuggestedFix != out.SuggestedFix {
+		t.Fatalf("string fields mismatch after round-trip")
+	}
+	if !reflect.DeepEqual(in.Evidence, out.Evidence) {
+		t.Fatalf("evidence mismatch after round-trip: in=%v out=%v", in.Evidence, out.Evidence)
+	}
+	if !in.FirstSeen.Equal(out.FirstSeen) || !in.LastSeen.Equal(out.LastSeen) {
+		t.Fatalf("time mismatch after round-trip: in=(%v,%v) out=(%v,%v)", in.FirstSeen, in.LastSeen, out.FirstSeen, out.LastSeen)
+	}
+}
+
+func TestJSON_InvalidEnumRejected(t *testing.T) {
+	// Priority invalid should be rejected.
+	var i Issue
+	if err := json.Unmarshal([]byte(`{"id":"x","category":"Storage","priority":"P9","title":"t","state":"Open","first_seen":"2025-12-20T12:00:00Z","last_seen":"2025-12-20T12:00:01Z"}`), &i); err == nil {
+		t.Fatalf("expected error for invalid priority")
+	}
+}