ci: add Docker cleanup before build to prevent disk space exhaustion (#473)

## Summary

Fixes #472

The Docker build job on the self-hosted runner fails with `no space left
on device` because Docker build cache and Go module downloads accumulate
between runs. The existing cleanup (line ~330) runs in the **deploy**
step *after* the build — too late to help.

## Changes

- Added a "Free disk space" step at the start of the build job,
**before** "Build Go Docker image":
- `docker system prune -af` — removes all unused images, containers,
networks
  - `docker builder prune -af` — clears the build cache
  - `df -h /` — logs available disk space for visibility
- Kept the existing post-deploy cleanup as belt-and-suspenders

---------

Co-authored-by: you <you@example.com>
Co-authored-by: Kpa-clawbot <kpabap+clawdbot@gmail.com>

.github/workflows/deploy.yml

@@ -246,6 +246,12 @@ jobs:
         with:
           node-version: '22'
 
+      - name: Free disk space
+        run: |
+          docker system prune -af 2>/dev/null || true
+          docker builder prune -af 2>/dev/null || true
+          df -h /
+
       - name: Build Go Docker image
         run: |
           echo "${GITHUB_SHA::7}" > .git-commit