chore(fork): drop private ee submodule and retarget CI to GHCR

Remove the private apps/server/src/ee git submodule (github.com/docmost/ee)
and the now-empty .gitmodules so that `git clone --recurse-submodules` and CI
checkout no longer fail with 404. The server loads EE only via guarded runtime
require(), so the build succeeds without it (community edition).

Rewrite .github/workflows/release.yml for the fork:
- drop the GitHub App token step and `submodules: recursive` checkout
- publish to GHCR (ghcr.io/vvzvlad/gitmost) via the built-in GITHUB_TOKEN
  instead of Docker Hub (docmost/docmost) — no extra secrets required
- add `packages: write` permission and an IMAGE env var
- log in as github.repository_owner; rename release tarballs to gitmost-*

Repoint the Dockerfile image source label to the fork.

Dockerfile

@@ -1,5 +1,5 @@
 FROM node:22-slim AS base
-LABEL org.opencontainers.image.source="https://github.com/docmost/docmost"
+LABEL org.opencontainers.image.source="https://github.com/vvzvlad/gitmost"
 
 RUN npm install -g pnpm@10.4.0