feat: add universal install script for macOS & Linux

- Add install.sh with curl one-liner installation
- Supports macOS (Homebrew), Debian/Ubuntu (apt), Fedora (dnf), Arch (pacman), Alpine (apk)
- Auto-detects OS, architecture, and Linux distro
- Checks/installs Node.js 18+, GNU Screen, warns about Claude CLI
- Clones to ~/.claudeman/app, builds, adds to PATH
- Optional systemd service setup on Linux
- Add postinstall.js for npm install verification
- Update README with simplified installation section

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

scripts/postinstall.js

@@ -0,0 +1,87 @@
+#!/usr/bin/env node
+
+import { execSync } from 'child_process';
+import { existsSync } from 'fs';
+import { homedir } from 'os';
+import { join } from 'path';
+
+// Colors (with fallback for no-color environments)
+const useColor = process.stdout.isTTY && !process.env.NO_COLOR;
+const green = (s) => useColor ? `\x1b[32m${s}\x1b[0m` : s;
+const yellow = (s) => useColor ? `\x1b[33m${s}\x1b[0m` : s;
+const red = (s) => useColor ? `\x1b[31m${s}\x1b[0m` : s;
+const bold = (s) => useColor ? `\x1b[1m${s}\x1b[0m` : s;
+
+console.log(bold('Claudeman postinstall check...'));
+
+let hasWarnings = false;
+
+// 1. Check Node.js version >= 18
+const nodeVersion = process.versions.node;
+const majorVersion = parseInt(nodeVersion.split('.')[0], 10);
+
+if (majorVersion < 18) {
+  console.log(red(`✗ Node.js v${nodeVersion} is too old (requires >=18)`));
+  process.exit(1);
+} else {
+  console.log(green(`✓ Node.js v${nodeVersion}`) + ' (meets >=18 requirement)');
+}
+
+// 2. Check if GNU Screen is installed
+let screenFound = false;
+try {
+  execSync('which screen', { stdio: 'pipe' });
+  screenFound = true;
+  console.log(green('✓ GNU Screen found'));
+} catch {
+  hasWarnings = true;
+  console.log(yellow('⚠ GNU Screen not found'));
+  console.log('  Install:');
+  console.log('    Ubuntu/Debian: sudo apt install screen');
+  console.log('    macOS: brew install screen');
+}
+
+// 3. Check if Claude CLI is found
+const home = homedir();
+const claudePaths = [
+  join(home, '.local/bin/claude'),
+  join(home, '.claude/local/claude'),
+  '/usr/local/bin/claude',
+  join(home, '.npm-global/bin/claude'),
+  join(home, 'bin/claude'),
+];
+
+let claudeFound = false;
+
+// First try `which claude`
+try {
+  execSync('which claude', { stdio: 'pipe' });
+  claudeFound = true;
+} catch {
+  // Check known paths
+  for (const p of claudePaths) {
+    if (existsSync(p)) {
+      claudeFound = true;
+      break;
+    }
+  }
+}
+
+if (claudeFound) {
+  console.log(green('✓ Claude CLI found'));
+} else {
+  hasWarnings = true;
+  console.log(yellow('⚠ Claude CLI not found'));
+  console.log('  Install: curl -fsSL https://claude.ai/install.sh | bash');
+}
+
+// Print next steps
+console.log('');
+console.log(bold('Next steps:'));
+console.log('  npm run build');
+console.log('  claudeman web');
+
+if (hasWarnings) {
+  console.log('');
+  console.log(yellow('Note: Resolve warnings above for full functionality.'));
+}