Improve action reliability for failed diff checks

[ajvalenty]

Summary

• default MATCH=0 at the start of the diff-check step before any git operations
• only set MATCH=1 after a full successful comparison confirms no change
• move SHA write/upload after the check step and gate both on steps.check.outcome == 'success'

Why

• if check logic fails mid-step, dismissal remains the safe default (MATCH=0)
• failed checks no longer overwrite SHA baseline artifacts, so subsequent runs compare from the last successful baseline

Validation

• parsed action.yml as YAML
• ran targeted assertions to verify check/upload ordering and success gating