chore: rename justfile -> Justfile, add flake.lock (estate-wide capitalization sweep)

Part of the 2026-04-17 estate-wide standardization on capitalized Justfile (matching standards/Justfile and rsr-template-repo). The file content is preserved; only the filename casing is changed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath