refactor(release): include docs/package.json in RELEASE_FILES

[Chemaclass]

🤔 Background

After #649 the docs package.json exists alongside the root one, but release.sh referenced it inline (two git add ... docs/package.json duplications) while leaving it out of the RELEASE_FILES array. That meant backup/rollback never covered it, and adding it naïvely would have collided with the root file because the backup helper flattened paths.

💡 Changes

• Promote docs/package.json to a first-class RELEASE_FILES entry so preflight, backup, rollback and commit-staging all handle it uniformly
• Drop the two inline git add ... docs/package.json duplications
• Update release::backup::save_file and release::rollback::restore_files to preserve nested directory structure
• New tests covering subdirectory backup + nested-path rollback