chore: gitignore worktree build directories

Worktree builds land in .build-wt-<hash>/ (177 MB in the current tree) and
were untracked but not ignored.

.gitignore

@@ -1,5 +1,6 @@
 .direnv
 .pio
+.build-wt-*
 .vscode/.browse.c_cpp.db*
 .vscode/c_cpp_properties.json
 .vscode/launch.json