CI: Set TZ=UTC for deterministic prompt signatures

The formatTimestamp() function uses getHours() which returns local time.
This caused different prompt signatures between local dev (Pacific/Auckland)
and CI (UTC), breaking fixture cache lookups.

Regenerated fixtures with TZ=UTC prompt signature.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ndbroadbent

.github/workflows/ci.yml

@@ -61,11 +61,13 @@ jobs:
         run: bun run test
         env:
           CI: true
+          TZ: UTC
 
       - name: Run E2E tests
         run: bun run vitest run --config src/cli/e2e/vitest.config.ts src/cli/e2e/
         env:
           CI: true
+          TZ: UTC
 
       - name: Build
         run: bun run build