fix: validate timezone to prevent crash on POSIX-style TZ values

POSIX-style TZ strings like IST-2 cause a hard RangeError crash in
formatMessages because Intl.DateTimeFormat only accepts IANA identifiers.

- Add isValidTimezone/resolveTimezone helpers to src/timezone.ts
- Make formatLocalTime fall back to UTC on invalid timezone
- Validate TZ candidates in config.ts before accepting
- Add timezone setup step to detect and prompt when autodetection fails
- Use node:22-slim in Dockerfile (node:24-slim Trixie package renames)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

setup/timezone.ts

@@ -0,0 +1,67 @@
+/**
+ * Step: timezone — Detect, validate, and persist the user's timezone.
+ * Writes TZ to .env if a valid IANA timezone is resolved.
+ * Emits NEEDS_USER_INPUT=true when autodetection fails.
+ */
+import fs from 'fs';
+import path from 'path';
+
+import { isValidTimezone } from '../src/timezone.js';
+import { logger } from '../src/logger.js';
+import { emitStatus } from './status.js';
+
+export async function run(args: string[]): Promise<void> {
+  const projectRoot = process.cwd();
+  const envFile = path.join(projectRoot, '.env');
+
+  // Check what's already in .env
+  let envFileTz: string | undefined;
+  if (fs.existsSync(envFile)) {
+    const content = fs.readFileSync(envFile, 'utf-8');
+    const match = content.match(/^TZ=(.+)$/m);
+    if (match) envFileTz = match[1].trim().replace(/^["']|["']$/g, '');
+  }
+
+  const systemTz = Intl.DateTimeFormat().resolvedOptions().timeZone;
+  const envTz = process.env.TZ;
+
+  // Accept --tz flag from CLI (used when setup skill collects from user)
+  const tzFlagIdx = args.indexOf('--tz');
+  const userTz = tzFlagIdx !== -1 ? args[tzFlagIdx + 1] : undefined;
+
+  // Resolve: user-provided > .env > process.env > system autodetect
+  let resolvedTz: string | undefined;
+  for (const candidate of [userTz, envFileTz, envTz, systemTz]) {
+    if (candidate && isValidTimezone(candidate)) {
+      resolvedTz = candidate;
+      break;
+    }
+  }
+
+  const needsUserInput = !resolvedTz;
+
+  if (resolvedTz && resolvedTz !== envFileTz) {
+    // Write/update TZ in .env
+    if (fs.existsSync(envFile)) {
+      let content = fs.readFileSync(envFile, 'utf-8');
+      if (/^TZ=/m.test(content)) {
+        content = content.replace(/^TZ=.*$/m, `TZ=${resolvedTz}`);
+      } else {
+        content = content.trimEnd() + `\nTZ=${resolvedTz}\n`;
+      }
+      fs.writeFileSync(envFile, content);
+    } else {
+      fs.writeFileSync(envFile, `TZ=${resolvedTz}\n`);
+    }
+    logger.info({ timezone: resolvedTz }, 'Set TZ in .env');
+  }
+
+  emitStatus('TIMEZONE', {
+    SYSTEM_TZ: systemTz || 'unknown',
+    ENV_TZ: envTz || 'unset',
+    ENV_FILE_TZ: envFileTz || 'unset',
+    RESOLVED_TZ: resolvedTz || 'none',
+    NEEDS_USER_INPUT: needsUserInput,
+    STATUS: needsUserInput ? 'needs_input' : 'success',
+  });
+}