README: the registry example named the wrong org

`api/packages/tools/rubygems` was right until the org reshuffle moved the engine
to `engines`, and it has been copy-pasteable-but-broken since. The gemspec's
allowed_push_host and the CI's `gem push --host` both say `engines`; only the
README did not.

While there: pin the example to `~> 0.5` and say why the source is block-scoped.
A second *global* source leaves Bundler unable to say which gem came from where.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

libs/ruby/warp_engine/README.md

@@ -161,14 +161,19 @@ From the git repository:
 gem "warp_engine", git: "https://git.teletypegames.org/engines/warp_engine.git"
 ```
 
-Or from the Forgejo rubygems registry (tagged releases):
+Or from the Forgejo rubygems registry, which is where **tagged releases** land — a
+version rather than whatever a branch happens to hold:
 
 ```ruby
-source "https://git.teletypegames.org/api/packages/tools/rubygems" do
-  gem "warp_engine"
+source "https://git.teletypegames.org/api/packages/engines/rubygems" do
+  gem "warp_engine", "~> 0.5"
 end
 ```
 
+The registry is publicly readable, so no credential is needed to install from it. Use a
+**block-scoped** source rather than a second global one: with two global sources Bundler
+cannot say which gem came from where.
+
 Then:
 
 ```sh