security(kdo): harden evidence-store access and retention for lossless request snapshots

[TheHalfMoon]

Context

KDO-H2-R1 intentionally persists a lossless model.request.snapshot so the exact provider-boundary model/messages/tools request is reconstructable.

A review of PR #45 correctly highlighted that durable request evidence increases the sensitivity of the event store. Redacting the snapshot itself is not an acceptable H2-R1 fix because it would violate the authorized lossless reconstruction invariant.

Follow-up scope

Design and authorize evidence-store protections independently of H2-R1 request semantics, including:

• restrictive file and directory permissions where the platform supports them;
• retention / expiry policy;
• deletion and cleanup behavior;
• explicit storage-location and access-policy documentation;
• threat model for repository prompts that may contain sensitive material;
• migration behavior for already-created evidence files;
• cross-platform behavior, especially Windows vs POSIX permissions;
• tests proving storage hardening does not alter the exact model-visible snapshot bytes/identity.

Non-goals

• Do not redact, digest, truncate, or otherwise weaken the H2-R1 lossless model-visible request snapshot.
• Do not change provider request semantics.
• Do not grant new K2 execution authority.

Entry condition

This requires a separate authorization/scope decision after H2-R1 PR #45 is reconciled. Until then, the issue is tracking-only.