Number section files to match reading order ...

sections/*.tex renamed to 1_introduction.tex .. 6_conclusion.tex so the
directory listing sorts in the same order they're \input from main.tex.
No 0_abstract.tex: the abstract must stay inline in main.tex (mdpi.cls
reuses it as PDF pdfsubject metadata, which \input breaks), and a
drafting-only file would just duplicate it with a risk of drifting out
of sync, so CLAUDE.md now says so explicitly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>