Fix docs cleanup shell quoting

[ChrisRackauckas-Claude]

Ignore this PR until it has been reviewed by @ChrisRackauckas.

Summary

• Quote the commit ID returned by git commit-tree before passing it to git branch.
• Quote the HEAD^{tree} revision expression.
• Remove the repo-wide actionlint SC2046 and SC1083 findings without changing cleanup behavior.

Root cause

The faulty cleanup workflow was introduced in 3a43dd9f0e5b4150dcd1aa3d81215ccd83b604bb (First version). Direct history inspection confirms the file is absent from its parent and is introduced with the unquoted command substitution and revision expression.

Local verification

• actionlint from the repository root: passed.
• git diff --check: passed.
• /home/crackauc/.juliaup/bin/julialauncher +1.12 --startup-file=no -m Runic --check .: passed.
• Executed the exact rewritten command in a temporary Git repository and verified the created branch has the same tree as HEAD and commit subject delete history: passed.