fix(P1): free disk space before Install CoreCI — runner disk full

The runner's disk is completely full from previous failed runs (Go
module cache ~500MB per run for coreci). Error:
  no space left on device
  write /root/go/pkg/mod/cache/download/...: no space left on device

Fix: add a 'Free disk space' step that removes /root/go/pkg/mod,
/root/.cache/go-build, and /tmp/coreci from previous runs before
installing CoreCI.

---ci---
project: orca
phase: 1
milestone: v0.16
status: execute
---/ci---

.gitea/workflows/release.yml

@@ -17,6 +17,11 @@ jobs:
         with:
           go-version: '1.25'
 
+      - name: Free disk space
+        run: |
+          rm -rf /root/go/pkg/mod /root/.cache/go-build /tmp/coreci 2>/dev/null || true
+          df -h /
+
       - name: Install CoreCI
         env:
           GITEA_TOKEN: ${{ secrets.PAT_TOKEN }}