plan/10: Add /api/stats endpoint and dashboard stats badge (#2)

Adds GET /api/stats endpoint with agent/plan/task/queue counts, stats widget in dashboard header, and Go test coverage.

ui/src/components/DashboardHeader.tsx

@@ -0,0 +1,18 @@
+import { ClaudeModeToggle } from './ClaudeModeToggle'
+import { StatsWidget } from './StatsWidget'
+import { NewPlanModal } from './NewPlanModal'
+
+export function DashboardHeader() {
+  return (
+    <header className="flex items-center justify-between border-b border-gray-200 dark:border-gray-700 px-4 py-3">
+      <div className="flex items-center gap-4">
+        <h1 className="text-xl font-serif font-semibold text-gray-900 dark:text-gray-100">Director</h1>
+        <NewPlanModal />
+      </div>
+      <div className="flex items-center gap-4">
+        <StatsWidget />
+        <ClaudeModeToggle />
+      </div>
+    </header>
+  )
+}

ui/src/components/StatsWidget.tsx

@@ -0,0 +1,63 @@
+import { useEffect, useState } from 'react'
+
+interface Stats {
+  status: string
+  agent_count: number
+  plan_counts: Record<string, number>
+  task_counts: Record<string, number>
+  queue_counts: Record<string, number>
+}
+
+export function StatsWidget() {
+  const [stats, setStats] = useState<Stats | null>(null)
+  const [error, setError] = useState<string | null>(null)
+
+  useEffect(() => {
+    const fetchStats = async () => {
+      try {
+        const response = await fetch('/api/stats')
+        if (!response.ok) {
+          throw new Error(`HTTP error! status: ${response.status}`)
+        }
+        const data = await response.json()
+        setStats(data)
+      } catch (err) {
+        setError(err instanceof Error ? err.message : 'Failed to fetch stats')
+      }
+    }
+
+    fetchStats()
+    const interval = setInterval(fetchStats, 30000) // Refresh every 30 seconds
+    return () => clearInterval(interval)
+  }, [])
+
+  if (error) {
+    return (
+      <div className="font-mono text-[10px] uppercase tracking-wider text-red-500">
+        err
+      </div>
+    )
+  }
+
+  if (!stats) {
+    return (
+      <div className="font-mono text-[10px] uppercase tracking-wider text-gray-400">
+        loading...
+      </div>
+    )
+  }
+
+  const activeAgents = stats.agent_count
+  const inProgressPlans = stats.plan_counts.in_progress || 0
+  const completedTasks = stats.task_counts.complete || 0
+
+  return (
+    <div className="flex items-center gap-1 font-mono text-[10px] uppercase tracking-wider text-gray-400">
+      <span className="text-gray-900 dark:text-gray-100">{activeAgents}</span>
+      <span>·</span>
+      <span className="text-gray-900 dark:text-gray-100">{inProgressPlans}</span>
+      <span>·</span>
+      <span className="text-gray-900 dark:text-gray-100">{completedTasks}</span>
+    </div>
+  )
+}