docs: preserve full session-tree audit

[nothingnesses]

Summary

• Preserve the sanitised full pre-reset session-tree audit under docs/audits/.
• Identify the immutable audited session prefix by boundary, byte size, and SHA-256 digest.
• Keep raw session extracts, maps, transcripts, and temporary analyses ignored.
• Select historical decision revalidation as the next bounded action.

Verification

• just ci.
• Independent product review found one mutable-source provenance defect.
• Separate triage confirmed the finding.
• Focused verification reproduced the corrected immutable prefix.