chore: untrack workspace/pi gitlink (no .gitmodules)

workspace/pi was tracked as a gitlink (mode 160000) with no
.gitmodules entry, causing actions/checkout post-cleanup to fail:
'No url found for submodule path workspace/pi in .gitmodules'.
It is a local dev checkout (CI clones pi fresh at build time), so
untrack it and add to .gitignore for consistency with the other
workspace symlinks.

Author: localpibox

.gitignore

@@ -52,3 +52,6 @@ __pycache__/
 
 # Backup of all localpibox repos before rebase operations
 backup/
+
+# Local Pi monorepo dev checkout — untracked (CI clones pi fresh at build time)
+workspace/pi/