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/ui/help.go

@@ -0,0 +1,152 @@
+package ui
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/charmbracelet/bubbles/key"
+)
+
+// HelpModel is the help overlay model.
+type HelpModel struct {
+	visible bool
+}
+
+// NewHelp creates a new help model.
+func NewHelp() HelpModel {
+	return HelpModel{
+		visible: false,
+	}
+}
+
+// Show displays the help overlay.
+func (m *HelpModel) Show() {
+	m.visible = true
+}
+
+// Hide hides the help overlay.
+func (m *HelpModel) Hide() {
+	m.visible = false
+}
+
+// Toggle toggles the help overlay visibility.
+func (m *HelpModel) Toggle() {
+	m.visible = !m.visible
+}
+
+// IsVisible returns true if the help overlay is visible.
+func (m HelpModel) IsVisible() bool {
+	return m.visible
+}
+
+// Render renders the help overlay.
+func (m HelpModel) Render(keys KeyMap, styles Styles) string {
+	if !m.visible {
+		return ""
+	}
+
+	var b strings.Builder
+
+	// Title
+	title := styles.HeaderBar.Render("Keybindings - Press ? or esc to close")
+	b.WriteString(title)
+	b.WriteString("\n\n")
+
+	// Define keybinding groups
+	groups := []struct {
+		name  string
+		binds []keyHelp
+	}{
+		{
+			name: "Global",
+			binds: []keyHelp{
+				{keys.Help, "Show/hide this help"},
+				{keys.Quit, "Quit the application"},
+				{keys.RefreshNow, "Refresh data now"},
+			},
+		},
+		{
+			name: "Filters",
+			binds: []keyHelp{
+				{keys.Search, "Search by title/details"},
+				{keys.Priority, "Cycle priority filter"},
+				{keys.Category, "Cycle category filter"},
+			},
+		},
+		{
+			name: "Navigation",
+			binds: []keyHelp{
+				{keys.FocusNext, "Toggle focus (table/details)"},
+				{keys.Sort, "Cycle sort order"},
+				{keys.JumpToTop, "Jump to top (g)"},
+				{keys.JumpToBottom, "Jump to bottom (G)"},
+				{keys.Down, "Move down (j)"},
+				{keys.Up, "Move up (k)"},
+			},
+		},
+		{
+			name: "Actions",
+			binds: []keyHelp{
+				{keys.AckToggle, "Acknowledge/unacknowledge issue"},
+				{keys.Export, "Export issues to JSON"},
+			},
+		},
+	}
+
+	// Render each group
+	for i, group := range groups {
+		if i > 0 {
+			b.WriteString("\n")
+		}
+
+		// Group header
+		groupTitle := styles.HeaderKey.Render(group.name + ":")
+		b.WriteString(groupTitle)
+		b.WriteString("\n")
+
+		// Keybindings in this group
+		for _, kb := range group.binds {
+			line := renderKeyHelp(kb, styles)
+			b.WriteString(line)
+			b.WriteString("\n")
+		}
+	}
+
+	// Render collector health icon legend
+	b.WriteString("\n")
+	legendTitle := styles.HeaderKey.Render("Legend:")
+	b.WriteString(legendTitle)
+	b.WriteString("\n")
+	legendText := styles.HeaderVal.Render("  Collector health: ✓ (OK), ⚠ (DEGRADED), ✗ (ERROR)")
+	b.WriteString(legendText)
+	b.WriteString("\n")
+
+	return b.String()
+}
+
+type keyHelp struct {
+	binding key.Binding
+	help    string
+}
+
+func renderKeyHelp(kb keyHelp, styles Styles) string {
+	// Get key names from the binding
+	keys := kb.binding.Keys()
+	if len(keys) == 0 {
+		return ""
+	}
+
+	// Format key names
+	keyStr := strings.Join(keys, ", ")
+	keyStyled := styles.HeaderVal.Render(keyStr)
+
+	// Format help text
+	helpStyled := styles.HeaderVal.Render(kb.help)
+
+	// Combine with padding
+	padding := ""
+	if needed := 10 - len(keyStr); needed > 0 {
+		padding = strings.Repeat(" ", needed)
+	}
+	return fmt.Sprintf("  %s%s%s", keyStyled, padding, helpStyled)
+}