Harden dogfooding diagnostic path redaction

[Chris0Jeky]

Summary

• extract dogfooding diagnostic path redaction into a path-flavour-aware, record-safe helper
• preserve home-relative output while handling POSIX case sensitivity, Windows drive/UNC roots, sibling prefixes, and home=/ correctly
• add table-driven unittest coverage without importing or executing the snapshot command

Verification

• red-first unittest run failed on the baseline because redact was not injectable for path-flavour fixtures
• py -3 -m unittest scripts/dogfooding/test_dogfood_snapshot.py -v: 4 tests passed
• py -3 scripts/dogfooding/dogfood-snapshot.py --help: passed
• py -3 -m compileall -q scripts/dogfooding/dogfood-snapshot.py scripts/dogfooding/test_dogfood_snapshot.py: passed
• git diff --check: passed

This is only the bounded redaction-helper row from #1480. Active-day definition, fixture classification, account attribution, and database precedence remain out of scope.

Refs #1480

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, add credits to your account and enable them for code reviews in your settings.