Limit test workflow PR triggers to main branch only

Previously these workflows ran on PRs to any branch. Now:
- PRs to main: run all tests (full CI gate)
- Pushes to other branches: use existing path filtering

This reduces CI load on feature branch PRs while maintaining
full test coverage for PRs targeting main.

Affected workflows:
- test-k8s-deploy.yml
- test-k8s-deployment-control.yml
- test-webapp.yml
- test-deploy.yml

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitea/workflows/test-deploy.yml

@@ -2,7 +2,8 @@ name: Deploy Test
 
 on:
   pull_request:
-    branches: '*'
+    branches:
+      - main
   push:
     branches:
       - main

.gitea/workflows/test-k8s-deploy.yml

@@ -2,7 +2,8 @@ name: K8s Deploy Test
 
 on:
   pull_request:
-    branches: '*'
+    branches:
+      - main
   push:
     branches: '*'
     paths:

.gitea/workflows/test-k8s-deployment-control.yml

@@ -2,7 +2,8 @@ name: K8s Deployment Control Test
 
 on:
   pull_request:
-    branches: '*'
+    branches:
+      - main
   push:
     branches: '*'
     paths:

.gitea/workflows/test-webapp.yml

@@ -2,7 +2,8 @@ name: Webapp Test
 
 on:
   pull_request:
-    branches: '*'
+    branches:
+      - main
   push:
     branches:
       - main