perf(memory-core): gate the recall HEAD probe on git ref file stats

RecallCorpusCache.load spawned `git rev-parse --verify HEAD` on every recall
trigger. It now fingerprints .git/HEAD (content and stat), the branch ref file
it points at and packed-refs, and reuses the cached revision while that
fingerprint holds. A .git file instead of a directory, a missing or unreadable
HEAD, or an unexpected ref name falls open to the git call exactly as before,
and head() is injectable so the probe is observable in tests.

Refs #8335