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>

MIGRATION.md

@@ -41,7 +41,8 @@ same shape the Nomarchy installer produces — see
 
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixos-25.11";
-    nomarchy.url = "github:bemagri/nomarchy";   # or pin to a commit/tag
+    nomarchy.url = "git+https://git.bemagri.xyz/bernardo/Nomarchy.git";
+    # …or pin: "git+https://git.bemagri.xyz/bernardo/Nomarchy.git?rev=<sha>"
     nixos-hardware.url = "github:NixOS/nixos-hardware/master";
     home-manager = {
       url = "github:nix-community/home-manager/release-25.11";
@@ -142,7 +143,7 @@ system:
 
 ```bash
 nix shell nixpkgs#git -c \
-  bash -c 'tmp=$(mktemp -d); git clone --depth=1 https://github.com/bemagri/nomarchy "$tmp" >/dev/null 2>&1; \
+  bash -c 'tmp=$(mktemp -d); git clone --depth=1 https://git.bemagri.xyz/bernardo/Nomarchy.git "$tmp" >/dev/null 2>&1; \
            source "$tmp/installer/hardware-db.sh"; nomarchy_detect_hw'
 ```