* test(#2864): add regression test for map-codebase date restamp
The wording fix shipped in #2550 without behavioral coverage, so the instruction could silently revert at any of the six date-stamp sites. This locks all six by reading the two shipped prompt files directly, using a per-file stale regex because the two files phrased the pre-fix instruction differently.
* test(#2864): cover both pre-fix phrasings in the workflow stale guard
The sequential-fallback site phrased the placeholder-only instruction with backticks and "from init context", so the stale-framing regex never matched it and that site's negative guard was dead. Only the site count caught a revert there.
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Co-authored-by: Tom Boucher <trekkie@nomorestars.com>
