fix(cron): preserve outputs from same-second runs

[lewismosciski]

Change Summary

Cron runs for the same job overwrite each other's saved output when they finish within one second. Allocate a unique Markdown file for each result, retaining a timestamp prefix and nanosecond ordering so get_output() can still read results chronologically.

Validation: three regressions failed before the fix; all 101 cron tests passed afterward. Tests cover sequential and concurrent execution, multiple executors with identical timestamps, existing files, UTF-8 content, and indexed/latest output reads. Applicable changed-file pre-commit hooks passed. Full-repository pre-commit reports existing main lint/format failures, including skills_manager.py:61 and shell_validator.py:91.

Related issue number

Found while inspecting cron output persistence; no existing issue.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Run pre-commit install and pre-commit run --all-files before git commit, and passed lint check. (Both run; existing repository failures noted above.)
• Documentation reflects the changes where applicable (internal persistence fix; no API change)