chore: stop tracking .worktrees gitlink

The .worktrees/modernization entry was an accidental submodule-style
gitlink (mode 160000) pointing into a local git worktree. There is no
.gitmodules and the worktree is not part of the repo tree, so it only
produced perpetual 'modified' noise in git status. Untrack it and
ignore .worktrees/ going forward.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jdpigeon

.gitignore

@@ -31,3 +31,6 @@ src/renderer/utils/webworker/src
 
 # Pyodide runtime + package wheels (downloaded or extracted locally; see docs/pyodide-in-electron-vite.md)
 src/renderer/utils/pyodide/src
+
+# Local git worktrees (not part of the repo tree)
+.worktrees/