chore: ignore a node_modules symlink, not only the directory

`node_modules/` with a trailing slash matches directories only, so a
worktree that links node_modules to a shared install shows the link as
untracked and a plain `git add` commits it. Two PR heads did (#658,
#662), which also broke checking them out next to a real install.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -8,8 +8,9 @@ DECISIONS.md
 # Written by install.sh into end-user clones when setup finishes
 .install-complete
 
-# Dependencies
-node_modules/
+# Dependencies. No trailing slash: `node_modules/` matches only a directory, so a
+# worktree's `node_modules` symlink to a shared install got committed (#658, #662).
+node_modules
 
 # Build output
 dist/