Add missing RBS annotation and signature for App#files_root, fixing the
two type errors that were blocking a clean steep run. Wire steep check
into the CI test job so type errors are caught on every push.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BFHmLzPvX1m71Qvu7ChjKb

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5694ffe..bc1fd1b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -27,6 +27,8 @@ jobs:
 
       - run: bundle exec rake test
 
+      - run: bundle exec steep check
+
   build:
     name: Build
     needs: test
diff --git a/lib/app.rb b/lib/app.rb
index 59b1247..7419a30 100644
--- a/lib/app.rb
+++ b/lib/app.rb
@@ -19,6 +19,7 @@ module Domus
       files_root / "#{record[:id]}#{record[:extension]}"
     end
 
+    # : () -> Pathname
     def files_root = config.storage_path / "files"
   end
 end
diff --git a/sig/generated/app.rbs b/sig/generated/app.rbs
index e3d8ddf..5010dc8 100644
--- a/sig/generated/app.rbs
+++ b/sig/generated/app.rbs
@@ -11,5 +11,8 @@ module Domus
 
     # : (Hash[Symbol, untyped]) -> Pathname
     def file_path: (untyped record) -> untyped
+
+    # : () -> Pathname
+    def files_root: () -> Pathname
   end
 end