remove bbolt integrity check entirely (release/v2)

[Trent-TSE]

Summary

release/v2 counterpart of #115: drops the bbolt integrity check entirely per discussion.

The check's RuntimeMaxSec=6s cap SIGTERMs (exit 143) bbolt check on large cache.db files, which was misread as corruption and skipped the sticky disk commit on every run, freezing the builder cache. The check was added for the copy-up corruption bug fixed in BLA-3252 and no longer fires legitimately.

• delete checkBoltDbIntegrity and its commit-gating in the post step
• delete reporter.reportIntegrityCheckFailure
• skip-integrity-check input kept in action.yml (marked deprecated, no effect)
• post-step logDatabaseHashes retained, relabeled "before unmount"

dist/ rebuilt with pnpm build.

Link to Devin session: https://app.devin.ai/sessions/35c21220e53a4e43a8287135eba4351d
Requested by: @Trent-TSE

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