fix: point downstream flakes at git.bemagri.xyz, not github

Upstream Nomarchy is hosted on the self-hosted Gitea at
git.bemagri.xyz/bernardo/Nomarchy.git, not github.com/bemagri/nomarchy.

- installer/install.sh: generated `nomarchy.url` now uses
  `git+https://git.bemagri.xyz/bernardo/Nomarchy.git` (with `?rev=<sha>`
  for the pinned form).
- MIGRATION.md: matches; the `hardware_detect` clone snippet now points
  at the same URL.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

installer/install.sh

@@ -736,11 +736,13 @@ generate_flake_config() {
     # Pin the upstream Nomarchy flake to the exact commit we're installing
     # from so the first post-reboot `nixos-rebuild` doesn't silently pull a
     # newer main. Fall back to tracking main if we couldn't resolve a SHA.
+    # Upstream lives on the self-hosted Gitea at git.bemagri.xyz; flakes
+    # consume it via the `git+https://` URL form.
     local nomarchy_url
     if [[ -n "$NOMARCHY_REV" ]]; then
-        nomarchy_url="github:bemagri/nomarchy/$NOMARCHY_REV"
+        nomarchy_url="git+https://git.bemagri.xyz/bernardo/Nomarchy.git?rev=$NOMARCHY_REV"
     else
-        nomarchy_url="github:bemagri/nomarchy"
+        nomarchy_url="git+https://git.bemagri.xyz/bernardo/Nomarchy.git"
     fi
 
     # flake.nix — the generator uses a non-quoted heredoc so $HOSTNAME,