ci(workflows): change kubeconfig from base64 to plain text

Remove base64 decoding from kubeconfig setup step.
Now accepts KUBE_CONFIG secret as plain text directly instead of base64 encoded.

This simplifies secret configuration in Gitea UI - users can paste kubeconfig
content directly without requiring base64 encoding.

Changes:
- deploy-prod.yml: Remove 'base64 -d' from kubeconfig setup
- deploy-alpha.yml: Remove 'base64 -d' from kubeconfig setup

Secret Configuration:
  Old: KUBE_CONFIG should be base64 encoded
  New: KUBE_CONFIG should be plain text kubeconfig content

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitea/workflows/deploy-alpha.yml

@@ -50,7 +50,7 @@ jobs:
       - name: Configure kubectl
         run: |
           mkdir -p ~/.kube
-          echo "${{ secrets.KUBE_CONFIG }}" | base64 -d > ~/.kube/config
+          echo "${{ secrets.KUBE_CONFIG }}" > ~/.kube/config
           chmod 600 ~/.kube/config
 
       - name: Validate kubeconfig and cluster connectivity

.gitea/workflows/deploy-prod.yml

@@ -50,7 +50,7 @@ jobs:
       - name: Configure kubectl
         run: |
           mkdir -p ~/.kube
-          echo "${{ secrets.KUBE_CONFIG }}" | base64 -d > ~/.kube/config
+          echo "${{ secrets.KUBE_CONFIG }}" > ~/.kube/config
           chmod 600 ~/.kube/config
 
       - name: Validate kubeconfig and cluster connectivity