refactor: rebase CI runner on Ubuntu 24.04, rename to ci-base

Replace Alpine-based legion-runner with a minimal Ubuntu 24.04 image.
The act_runner binary is copied from the official image via multi-stage
build. Job-specific tooling (Ruby, Node, Python, etc.) belongs in
per-job container images specified via container: in workflows, not
in the base runner image.

Rename: legion-runner → ci-base

servers/legion/apps/gitea-runner.yaml

@@ -17,7 +17,7 @@ spec:
     spec:
       initContainers:
         - name: register
-          image: registry.neuralplatform.ai/legion-runner:latest
+          image: registry.neuralplatform.ai/ci-base:latest
           workingDir: /data
           command: ["/bin/sh", "-c"]
           args:
@@ -47,7 +47,7 @@ spec:
               mountPath: /data
       containers:
         - name: runner
-          image: registry.neuralplatform.ai/legion-runner:latest
+          image: registry.neuralplatform.ai/ci-base:latest
           workingDir: /data
           command: ["act_runner", "daemon", "--config", "/data/config.yaml"]
           envFrom: