Add three optional reliability features for hardening CI pipelines:
- Git corruption detection & recovery (fsck, stale lock cleanup,
submodule backing store validation, auto-recovery)
- Reserved filename cleanup (removes Windows device names that
cause Unity asset importer infinite loops)
- Build output archival with configurable retention policy
All features are opt-in and fail gracefully with warnings only.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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