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>

container/Dockerfile

@@ -1,7 +1,7 @@
 # NanoClaw Agent Container
 # Runs Claude Agent SDK in isolated Linux VM with browser automation
 
-FROM node:24-slim
+FROM node:22-slim
 
 # Install system dependencies for Chromium
 RUN apt-get update && apt-get install -y \