fix: avoid using saveState to detect unclean buildkitd shutdown

[haines]

Fixes #68

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Note

Medium Risk
Moderate risk because it changes cleanup decision logic that gates sticky-disk commits; incorrect shutdown classification could skip commits or commit after a bad shutdown.

Overview
Cleanup now treats shutdownBuildkitd() as returning whether shutdown was graceful vs forced, and logs the outcome accordingly.

Sticky-disk commit gating is updated to skip commits whenever BuildKit did not shut down cleanly (including crashes/missing process), and the old sigkillUsed state flag is removed from state-helper/shutdown flow.

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