Standardize timestamp format across Fromager logs and artifacts #1318

Problem

Fromager currently emits internally generated timestamps in different formats and timezones:

• Log files use Python logging's local-time asctime format: YYYY-MM-DD HH:MM:SS,mmm, without a timezone marker.
• Test-mode report filenames use UTC in the format YYYYMMDD-HHMMSS-ffffff, but do not include an explicit timezone marker.
• Other generated artifacts may use additional timestamp formats.

This makes it difficult to correlate logs and reports, especially when log files or work-dir are reused across local runs. This was identified while reviewing #1314.

Goal

Find and implement a consistent timestamp strategy for Fromager's internally generated logs, filenames, and report metadata. The solution should use an explicit timezone, be suitable for both CI and local workflows, and make artifacts from the same run easy to correlate.

Investigation and possible directions

The right solution requires deeper investigation across Fromager's timestamp producers and consumers, including logging, generated artifacts, report filenames, run correlation, and backward compatibility.

Some initial ideas to evaluate:

• Define a canonical UTC ISO-8601 format with an explicit timezone marker.
• Introduce a shared timestamp or run-ID helper.
• Correlate bootstrap-start logs with generated reports.
• Update timestamped filenames, report metadata, tests, and documentation.
• Consider behavior when logs or work-dir are reused across runs.

These are starting points, not final implementation requirements. The implementation should follow the solution that best fits Fromager's CLI, CI, and local workflows.