docs: clarify Longmont proof chunk parameters

[scottconverse]

Summary

• Clarifies that the CivicCore Longmont proof used chunk_size=900 / chunk_overlap=90.
• Explains why CivicCore's 1,789 chunks do not match CivicCode PR ci: add CivicCore cleanroom release rehearsal #61's 2,931 chunks, which uses chunk_size=500 / chunk_overlap=50.
• Keeps this as documentation-only shared-dependency evidence cleanup; no runtime code, version, tag, or release claim changes.

Verification

• bash scripts/verify-release.sh -> PASS

Scope

Controlled shared-dependency exception for CivicCode PR #61 audit finding C2. Queued modules untouched.