feat(tui): add terminal user interface with Ink

Add a full TUI for Claudeman using Ink (React for CLI):
- StartScreen: Shows existing sessions from screens.json
- TabBar: Session tabs with status indicators
- TerminalView: PTY output display with scrolling
- StatusBar: Session info and keyboard shortcuts
- HelpOverlay: Full keyboard shortcut reference

Features:
- Session discovery from ~/.claudeman/screens.json
- Tab navigation (Ctrl+Tab, Ctrl+1-9)
- Create new sessions (connects to web API)
- Kill sessions via screen commands
- Keyboard-driven navigation

Usage: claudeman tui

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/tui/components/HelpOverlay.tsx

@@ -0,0 +1,99 @@
+/**
+ * @fileoverview HelpOverlay component
+ *
+ * Full-screen overlay showing all keyboard shortcuts.
+ * Dismissible with Escape, q, or ?.
+ */
+
+import React from 'react';
+import { Box, Text } from 'ink';
+
+interface HelpOverlayProps {
+  onClose: () => void;
+}
+
+interface ShortcutGroup {
+  title: string;
+  shortcuts: Array<{ key: string; description: string }>;
+}
+
+const SHORTCUT_GROUPS: ShortcutGroup[] = [
+  {
+    title: 'Navigation',
+    shortcuts: [
+      { key: 'Ctrl+Tab', description: 'Next session' },
+      { key: 'Ctrl+Shift+Tab', description: 'Previous session' },
+      { key: 'Ctrl+1-9', description: 'Go to session N' },
+      { key: 'Escape', description: 'Back to start screen' },
+    ],
+  },
+  {
+    title: 'Session Management',
+    shortcuts: [
+      { key: 'Ctrl+N', description: 'New session' },
+      { key: 'Ctrl+W', description: 'Close current session' },
+      { key: 'Ctrl+K', description: 'Kill all sessions' },
+    ],
+  },
+  {
+    title: 'Start Screen',
+    shortcuts: [
+      { key: '1-9', description: 'Select session by number' },
+      { key: 'n', description: 'Create new session' },
+      { key: 'r', description: 'Refresh session list' },
+      { key: 'q', description: 'Quit' },
+    ],
+  },
+  {
+    title: 'General',
+    shortcuts: [
+      { key: '?', description: 'Toggle this help' },
+      { key: 'Ctrl+C', description: 'Exit TUI' },
+    ],
+  },
+];
+
+/**
+ * HelpOverlay component showing keyboard shortcuts
+ */
+export function HelpOverlay({ onClose }: HelpOverlayProps): React.ReactElement {
+  return (
+    <Box flexDirection="column" padding={2}>
+      {/* Header */}
+      <Box
+        borderStyle="double"
+        borderColor="cyan"
+        paddingX={2}
+        paddingY={1}
+        marginBottom={1}
+        justifyContent="center"
+      >
+        <Text bold color="cyan">
+          Keyboard Shortcuts
+        </Text>
+      </Box>
+
+      {/* Shortcut groups */}
+      {SHORTCUT_GROUPS.map((group, groupIndex) => (
+        <Box key={groupIndex} flexDirection="column" marginBottom={1}>
+          <Text bold color="yellow">
+            {group.title}
+          </Text>
+          <Box flexDirection="column" marginLeft={2}>
+            {group.shortcuts.map((shortcut, index) => (
+              <Box key={index}>
+                <Text color="green">{shortcut.key.padEnd(20)}</Text>
+                <Text>{shortcut.description}</Text>
+              </Box>
+            ))}
+          </Box>
+        </Box>
+      ))}
+
+      {/* Footer */}
+      <Box marginTop={1} borderStyle="single" borderColor="gray" paddingX={1}>
+        <Text dimColor>Press Escape, q, or ? to close</Text>
+      </Box>
+    </Box>
+  );
+}