Identify and pin upstream source for vendored proto-installer

[hanthor-hive-agent]

The GNOME OS installer prototype vendored at files/kde-linux-system/live/proto-installer/ (installer.py, daemon/ Rust daemon, org.gnome.Installer* D-Bus/systemd/polkit units, UI/resource files) has no recorded provenance: no README (until the doc-only fix in #198), no .gitmodules, no upstream commit/tag reference, no sync workflow.

Per files/kde-linux-system/live/proto-installer/README.md (added in #198):

• The tree entered this repo's history in commit 15cf8a5 via a git subtree add --squash import of the retired tuna-os/kde-build-meta repo, which itself carried no provenance record for these files.
• Naming and structure (org.gnome.Installer D-Bus service, gnomeos-installer prefix, GTK4 + Adwaita UI, Rust daemon driving UDisks/logind/polkit/systemd) are consistent with a GNOME OS team installer prototype, but no specific upstream repo URL or commit hash has been confirmed.

Ask

• Track down the actual upstream project (likely a GNOME OS / gnome-os-installer prototype repo) and confirm the commit or tag this snapshot corresponds to.
• Once confirmed, either:
  • convert proto-installer/ to a git submodule pinned to that commit, or
  • record the pinned upstream commit/tag in the README and set up a sync workflow (e.g. periodic diff/rebase check) to detect drift.

This is a precondition for safely receiving upstream fixes (security, partitioning, TPM/verity handling) instead of the tree silently drifting.

Part of #196

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