ci(infra): switch runner label to ubuntu-latest (refs #30)

Self-hosted respellion-linux runner not required — Gitea's hosted
ubuntu-latest runner has Docker + Compose v2 out of the box, so
make smoke works without any manual registration step.

Updates docs/runbooks/ci.md to reflect the new runner label and
removes the act_runner self-hosted setup as the primary path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -17,7 +17,7 @@ permissions:
 
 jobs:
   lint:
-    runs-on: respellion-linux
+    runs-on: ubuntu-latest
     steps:
       - uses: https://github.com/actions/checkout@v4
       - uses: https://github.com/actions/setup-dotnet@v4
@@ -26,7 +26,7 @@ jobs:
       - run: make lint
 
   build:
-    runs-on: respellion-linux
+    runs-on: ubuntu-latest
     steps:
       - uses: https://github.com/actions/checkout@v4
       - uses: https://github.com/actions/setup-dotnet@v4
@@ -35,7 +35,7 @@ jobs:
       - run: make build
 
   unit:
-    runs-on: respellion-linux
+    runs-on: ubuntu-latest
     steps:
       - uses: https://github.com/actions/checkout@v4
       - uses: https://github.com/actions/setup-dotnet@v4
@@ -44,7 +44,7 @@ jobs:
       - run: make unit
 
   compose-smoke:
-    runs-on: respellion-linux
+    runs-on: ubuntu-latest
     steps:
       - uses: https://github.com/actions/checkout@v4
       - run: make smoke