Fix commit-pdfs: git add, artifact upload, DEV cleanup, no-op guard

[ViralBShah]

Summary

Fixes several issues in the commit() function and workflow that caused two consecutive failures on master (#45, #46):

Bug fixes

• git add splatting: $(files...) in a Julia Cmd concatenates filenames into one string (file1.pdffile2.pdf). Replaced with git add -A which correctly stages all changes
• .commit files not uploaded as artifacts: The upload pattern was *.pdf only, so nightly commit markers were lost between build and commit jobs. Added *.commit to the artifact upload
• Duplicate git push -f: Push was called twice — once inside the conditional, once unconditionally after. Removed the duplicate
• No-op push guard: If no files changed (all nightlies skipped), git commit --amend + git push -f would rewrite history unnecessarily. Now checks git diff --cached --quiet and skips if nothing to commit

New feature

• DEV PDF cleanup: When a release PDF exists (e.g. julia-1.12.6.pdf), automatically removes the corresponding julia-1.12.6-DEV.pdf and its .commit marker from the assets branch

Nightly safety

• Skip non-DEV nightlies: Release branch nightlies that just mirror the latest tag (e.g. 1.10 reporting 1.10.11 instead of 1.10.12-DEV) are now skipped to avoid overwriting release PDFs

Test plan

• commit-pdfs job succeeds (no more git add errors)
• Nightly .commit files appear on the assets branch
• No unnecessary force-pushes when nothing changed
• DEV PDFs removed when corresponding release exists

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