fix: format Claude prompt time in UTC

[vycdev2]

Summary

• format the current-time context in Claude response prompts explicitly in UTC
• add a regression test that runs under a non-UTC host timezone
• document the user-visible consistency fix in the changelog

Verification

• npm run build — passed
• npm test — passed (110 tests)
• node --test /opt/data/claudify/tests/promptTime.test.mjs from /tmp — passed
• regression test copied to untouched upstream/main — failed as expected with a PDT prompt timestamp
• git diff --check upstream/main...HEAD — passed

Risk

• Low: this only makes an existing prompt timestamp deterministic and aligns it with Claudify’s UTC history and live-context timestamps.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.