fix(pages): untrack dangling Mac-path symlinks that broke GitHub Pages

AGENTS.md, CLAUDE.md, jj_cheatsheet.md were tracked symlinks pointing at absolute
/Users/pmarreck/... paths (an Obsidian vault + dotfiles) that don't exist off the
Mac — dangling on this Linux box AND on the Pages runner, where the deploy-from-
branch tar step fails on them (exit 1). They're local-only conveniences (and
.gitignore already lists AGENTS*/CLAUDE*; policy says don't commit them), so
untrack (working-copy symlinks kept) and gitignore jj_cheatsheet.md.

Author: pmarreck

.gitignore

@@ -39,3 +39,6 @@ ZIG_0.15_TO_0.16_MIGRATION.md
 !bin/utf16to8
 AGENTS*.md
 CLAUDE*.md
+# Local convenience symlink to ~/dotfiles (external target — must not be committed
+# into a published repo; broke the GitHub Pages build as a dangling symlink).
+jj_cheatsheet.md