feat(setup): auto-detect timezone after CLI agent step

Adds a timezone step between cli-agent and channel wiring in setup:auto.
Autodetect via --step timezone; if it resolves to UTC or fails, confirm
with the user and accept either an IANA zone or a free-text description
(e.g. "New York"). Free-text falls through to a headless `claude -p`
call that returns a single IANA string, gated on the claude CLI being
on PATH.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

setup/lib/claude-assist.ts

@@ -56,6 +56,7 @@ const STEP_FILES: Record<string, string[]> = {
   mounts: ['setup/mounts.ts'],
   service: ['setup/service.ts'],
   'cli-agent': ['setup/cli-agent.ts', 'scripts/init-cli-agent.ts'],
+  timezone: ['setup/timezone.ts', 'setup/lib/tz-from-claude.ts'],
   channel: ['setup/auto.ts'],
   verify: ['setup/verify.ts'],
   // Channel-specific sub-steps: