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/collector.go

@@ -0,0 +1,45 @@
+package collectors
+
+import (
+	"context"
+	"time"
+
+	"tower/internal/model"
+)
+
+type Health string
+
+const (
+	HealthOK       Health = "OK"
+	HealthDegraded Health = "DEGRADED"
+	HealthError    Health = "ERROR"
+)
+
+// Status describes collector health for the current tick.
+//
+// Collectors should return Status even when returning an error,
+// so the UI can show useful context.
+//
+// LastSuccess should be the collector's most recent successful collect time.
+// When unknown, it may be the zero value.
+//
+// Message should be short and human-friendly.
+type Status struct {
+	Health      Health    `json:"health"`
+	Message     string    `json:"message,omitempty"`
+	LastSuccess time.Time `json:"last_success,omitempty"`
+}
+
+func OKStatus() Status {
+	return Status{Health: HealthOK}
+}
+
+// Collector returns "currently true" issues for this tick.
+//
+// The store is responsible for dedupe, lifecycle, and resolve-after.
+// Collectors must respect ctx cancellation.
+type Collector interface {
+	Name() string
+	Interval() time.Duration
+	Collect(ctx context.Context) ([]model.Issue, Status, error)
+}