Fix timezone-aware --since filtering in fetch-workflow-logs

[github-actions]

Summary

This fixes incorrect --since filtering in scripts/fetch-workflow-logs.py when the since value includes a timezone offset (for example, 2025-01-01T00:00:00-05:00).

Root cause: the script compared timestamps as raw strings, which is not chronologically correct across differing timezone representations.

What changed

• Replaced string-based boundary comparisons with timezone-aware datetime comparisons in UTC.
• Added _parse_iso8601_utc() to parse ISO 8601 timestamps (including Z and explicit offsets) and normalize to UTC.
• Added _normalize_since() for date-only --since values so they are interpreted as start-of-day UTC (T00:00:00Z), mirroring the existing date-only --until normalization.
• Added a regression test covering the reported case where 2025-01-01T04:30:00Z must be excluded for --since 2025-01-01T00:00:00-05:00.

Validation

UV_CACHE_DIR=/tmp/gh-aw/agent/uv-cache uv run --extra test pytest -q tests/test_fetch_workflow_logs.py
make lint

Closes #2158.

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