refactor: revert CI image approach and use standard npm ci in workflows

The CI image approach with pre-installed dependencies doesn't work with Gitea Actions due to workspace mounting issues. When Gitea Actions runs a container job, it mounts the workspace at a specific path (e.g., /workspace/david/euchre_camp), which hides the container's /app directory where dependencies were installed.

Changes:
- Reverted pr.yml unit-tests to use node:20-alpine with npm ci
- Updated pr.yml acceptance-tests to use mcr.microsoft.com/playwright image with npm ci
- Updated test.yml to use node:20-alpine with npm ci
- Removed ci-image-builder.yml workflow
- Removed Dockerfile.ci and build-ci-image.sh
- Updated AGENTS.md and README.md to document the deprecation
- Updated justfile to remove CI image commands

The standard approach of running npm ci in each workflow is the recommended approach for Gitea Actions.

.gitea/workflows/test.yml

@@ -9,21 +9,17 @@ jobs:
   unit-tests:
     runs-on: ubuntu-latest
     container:
-      image: docker.notsosm.art/euchre-camp-ci-runner:latest
+      image: node:20-alpine
       options: --user root
     # Skip if this is an auto-generated version bump commit (handled by release workflow)
     if: "!contains(github.event.head_commit.message, 'chore: bump version')"
-    env:
-      NODE_PATH: /workspace/david/euchre_camp/node_modules
 
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
 
-      - name: Link node_modules from container
-        run: |
-          # Create symlink from container's node_modules to workspace
-          ln -sf /app/node_modules /workspace/david/euchre_camp/node_modules
+      - name: Install dependencies
+        run: npm ci
 
       - name: Run unit tests
         run: npm run test:run