feat: add gsd sessions subcommand for session picker

Add a new `gsd sessions` subcommand that lists all saved sessions for
the current directory and lets the user interactively pick one to resume.

Currently `gsd --continue` only resumes the most recent session, with no
way to access older conversations. This change adds:

- `gsd sessions` subcommand that calls SessionManager.list() to enumerate
  all sessions for the current working directory
- Interactive numbered list showing date, message count, session name (if
  set), and a preview of the first message
- Selection by number to resume any past session via SessionManager.open()
- Subcommand help text (`gsd sessions --help`)
- Help text entry in the main `gsd --help` output

The implementation uses only existing SessionManager APIs (list, open) -
no SDK changes required.

src/cli.ts

@@ -35,6 +35,8 @@ interface CliFlags {
   appendSystemPrompt?: string
   tools?: string[]
   messages: string[]
+  /** Set by `gsd sessions` when the user picks a specific session to resume */
+  _selectedSessionPath?: string
 }
 
 function exitIfManagedResourcesAreNewer(currentAgentDir: string): void {
@@ -115,6 +117,63 @@ if (cliFlags.messages[0] === 'update') {
   process.exit(0)
 }
 
+// `gsd sessions` — list past sessions and pick one to resume
+if (cliFlags.messages[0] === 'sessions') {
+  const cwd = process.cwd()
+  const safePath = `--${cwd.replace(/^[/\\]/, '').replace(/[/\\:]/g, '-')}--`
+  const projectSessionsDir = join(sessionsDir, safePath)
+
+  process.stderr.write(chalk.dim(`Loading sessions for ${cwd}...\n`))
+  const sessions = await SessionManager.list(cwd, projectSessionsDir)
+
+  if (sessions.length === 0) {
+    process.stderr.write(chalk.yellow('No sessions found for this directory.\n'))
+    process.exit(0)
+  }
+
+  process.stderr.write(chalk.bold(`\n  Sessions (${sessions.length}):\n\n`))
+
+  const maxShow = 20
+  const toShow = sessions.slice(0, maxShow)
+  for (let i = 0; i < toShow.length; i++) {
+    const s = toShow[i]
+    const date = s.modified.toLocaleString()
+    const msgs = s.messageCount
+    const name = s.name ? ` ${chalk.cyan(s.name)}` : ''
+    const preview = s.firstMessage
+      ? s.firstMessage.replace(/\n/g, ' ').substring(0, 80)
+      : chalk.dim('(empty)')
+    const num = String(i + 1).padStart(3)
+    process.stderr.write(`  ${chalk.bold(num)}. ${chalk.green(date)} ${chalk.dim(`(${msgs} msgs)`)}${name}\n`)
+    process.stderr.write(`       ${chalk.dim(preview)}\n\n`)
+  }
+
+  if (sessions.length > maxShow) {
+    process.stderr.write(chalk.dim(`  ... and ${sessions.length - maxShow} more\n\n`))
+  }
+
+  // Interactive selection
+  const readline = await import('node:readline')
+  const rl = readline.createInterface({ input: process.stdin, output: process.stderr })
+  const answer = await new Promise<string>((resolve) => {
+    rl.question(chalk.bold('  Enter session number to resume (or q to quit): '), resolve)
+  })
+  rl.close()
+
+  const choice = parseInt(answer, 10)
+  if (isNaN(choice) || choice < 1 || choice > toShow.length) {
+    process.stderr.write(chalk.dim('Cancelled.\n'))
+    process.exit(0)
+  }
+
+  const selected = toShow[choice - 1]
+  process.stderr.write(chalk.green(`\nResuming session from ${selected.modified.toLocaleString()}...\n\n`))
+
+  // Mark for the interactive session below to open this specific session
+  cliFlags.continue = true
+  cliFlags._selectedSessionPath = selected.path
+}
+
 // 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.
@@ -350,9 +409,11 @@ if (existsSync(sessionsDir)) {
   }
 }
 
-const sessionManager = cliFlags.continue
-  ? SessionManager.continueRecent(cwd, projectSessionsDir)
-  : SessionManager.create(cwd, projectSessionsDir)
+const sessionManager = cliFlags._selectedSessionPath
+  ? SessionManager.open(cliFlags._selectedSessionPath, projectSessionsDir)
+  : cliFlags.continue
+    ? SessionManager.continueRecent(cwd, projectSessionsDir)
+    : SessionManager.create(cwd, projectSessionsDir)
 
 exitIfManagedResourcesAreNewer(agentDir)
 initResources(agentDir)

src/help-text.ts

@@ -18,6 +18,19 @@ const SUBCOMMAND_HELP: Record<string, string> = {
     '',
     'Equivalent to: npm install -g gsd-pi@latest',
   ].join('\n'),
+
+  sessions: [
+    'Usage: gsd sessions',
+    '',
+    'List all saved sessions for the current directory and interactively',
+    'pick one to resume. Shows date, message count, and a preview of the',
+    'first message for each session.',
+    '',
+    'Sessions are stored per-directory, so you only see sessions that were',
+    'started from the current working directory.',
+    '',
+    'Compare with --continue (-c) which always resumes the most recent session.',
+  ].join('\n'),
 }
 
 export function printHelp(version: string): void {
@@ -37,6 +50,7 @@ export function printHelp(version: string): void {
   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.stdout.write('  sessions                 List and resume a past session\n')
   process.stdout.write('\nRun gsd <subcommand> --help for subcommand-specific help.\n')
 }