Add CI split-mirror pipeline for WarpEngine

- .woodpecker.yaml: on master pushes touching libs/ruby/warp_engine, split the
  subtree and force-push it to the read-only tools/warp_engine mirror; on
  warp_engine-v* tags, build and push the gem to the Forgejo rubygems registry
- engine README documents the monorepo-first workflow and the mirror

Requires a `forge_token` Woodpecker secret (repository:write + package:write).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.woodpecker.yaml

@@ -0,0 +1,51 @@
+# Read-only split mirror: a libs/ruby/warp_engine alkönyvtárat kitükrözi a
+# tools/warp_engine repóba (fejlesztés itt, a monorepóban történik; a tükör
+# csak publikálásra való). Tag-elt release (warp_engine-v*) esetén a gem a
+# Forgejo rubygems registry-be is felmegy.
+#
+# Szükséges Woodpecker secret: forge_token — Forgejo access token
+# repository:write (tools/warp_engine) és package:write joggal.
+
+when:
+  - event: push
+    branch: master
+    path: "libs/ruby/warp_engine/**"
+  - event: tag
+    ref: refs/tags/warp_engine-v*
+
+clone:
+  git:
+    image: woodpeckerci/plugin-git
+    settings:
+      partial: false
+      depth: 0 # a subtree splithez teljes history kell
+
+steps:
+  split-mirror:
+    image: alpine/git
+    environment:
+      FORGE_TOKEN:
+        from_secret: forge_token
+    commands:
+      - apk add --no-cache git-subtree
+      - git subtree split --prefix=libs/ruby/warp_engine HEAD -b warp-engine-split
+      - git push --force "https://ci:$${FORGE_TOKEN}@git.teletypegames.org/tools/warp_engine.git" warp-engine-split:master
+    when:
+      - event: push
+        branch: master
+
+  publish-gem:
+    image: ruby:3.3-slim
+    environment:
+      FORGE_TOKEN:
+        from_secret: forge_token
+    commands:
+      - cd libs/ruby/warp_engine
+      - gem build warp_engine.gemspec
+      - mkdir -p ~/.gem
+      - printf -- '---\n:https://git.teletypegames.org/api/packages/tools/rubygems: Bearer %s\n' "$${FORGE_TOKEN}" > ~/.gem/credentials
+      - chmod 600 ~/.gem/credentials
+      - gem push --host https://git.teletypegames.org/api/packages/tools/rubygems warp_engine-*.gem
+    when:
+      - event: tag
+        ref: refs/tags/warp_engine-v*

libs/ruby/warp_engine/README.md

@@ -4,6 +4,13 @@ Mountable Rails engine: a retro software catalog with a CI-pipeline-callable
 release updater, a public read-only JSON API, and ActiveAdmin resources that
 load into the host application's admin.
 
+> **Development happens in the `tools/teletypegames` monorepo** under
+> `libs/ruby/warp_engine`. The standalone `tools/warp_engine` repository is a
+> **read-only split mirror** published by CI on every master push — do not
+> push to it directly. Tagged releases (`warp_engine-vX.Y.Z`) are also
+> published as a gem to the Forgejo rubygems registry
+> (`https://git.teletypegames.org/api/packages/tools/rubygems`).
+
 ## What it provides
 
 - **Models**: `Software`, `Release`, `ReleaseAsset`, `ExternalLink`,