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/index.ts

@@ -9,6 +9,7 @@ const STEPS: Record<
   string,
   () => Promise<{ run: (args: string[]) => Promise<void> }>
 > = {
+  timezone: () => import('./timezone.js'),
   environment: () => import('./environment.js'),
   container: () => import('./container.js'),
   groups: () => import('./groups.js'),