feat: add gsd update subcommand for self-update

Adds a CLI subcommand that checks npm for the latest version and
runs `npm install -g gsd-pi@latest` if an update is available.
Prints current/latest version and clear success/failure messages.

src/cli.ts

@@ -74,6 +74,7 @@ function parseCliArgs(argv: string[]): CliFlags {
       process.stdout.write('  --help, -h               Print this help and exit\n')
       process.stdout.write('\nSubcommands:\n')
       process.stdout.write('  config                   Re-run the setup wizard\n')
+      process.stdout.write('  update                   Update GSD to the latest version\n')
       process.exit(0)
     } else if (!arg.startsWith('--') && !arg.startsWith('-')) {
       flags.messages.push(arg)
@@ -92,6 +93,13 @@ if (cliFlags.messages[0] === 'config') {
   process.exit(0)
 }
 
+// `gsd update` — update to the latest version via npm
+if (cliFlags.messages[0] === 'update') {
+  const { runUpdate } = await import('./update-cmd.js')
+  await runUpdate()
+  process.exit(0)
+}
+
 // Pi's tool bootstrap can mis-detect already-installed fd/rg on some systems
 // because spawnSync(..., ["--version"]) returns EPERM despite a zero exit code.
 // Provision local managed binaries first so Pi sees them without probing PATH.

src/update-cmd.ts

@@ -0,0 +1,45 @@
+import { execSync } from 'node:child_process'
+import { compareSemver } from './update-check.js'
+
+const NPM_PACKAGE = 'gsd-pi'
+
+export async function runUpdate(): Promise<void> {
+  const current = process.env.GSD_VERSION || '0.0.0'
+  const bold = '\x1b[1m'
+  const dim = '\x1b[2m'
+  const green = '\x1b[32m'
+  const yellow = '\x1b[33m'
+  const reset = '\x1b[0m'
+
+  process.stdout.write(`${dim}Current version:${reset} v${current}\n`)
+  process.stdout.write(`${dim}Checking npm registry...${reset}\n`)
+
+  // Fetch latest version
+  let latest: string
+  try {
+    latest = execSync(`npm view ${NPM_PACKAGE} version`, {
+      encoding: 'utf-8',
+      stdio: ['ignore', 'pipe', 'ignore'],
+    }).trim()
+  } catch {
+    process.stderr.write(`${yellow}Failed to reach npm registry.${reset}\n`)
+    process.exit(1)
+  }
+
+  if (compareSemver(latest, current) <= 0) {
+    process.stdout.write(`${green}Already up to date.${reset}\n`)
+    return
+  }
+
+  process.stdout.write(`${dim}Updating:${reset} v${current} → ${bold}v${latest}${reset}\n`)
+
+  try {
+    execSync(`npm install -g ${NPM_PACKAGE}@latest`, {
+      stdio: 'inherit',
+    })
+    process.stdout.write(`\n${green}${bold}Updated to v${latest}${reset}\n`)
+  } catch {
+    process.stderr.write(`\n${yellow}Update failed. Try manually: npm install -g ${NPM_PACKAGE}@latest${reset}\n`)
+    process.exit(1)
+  }
+}