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.

justfile

@@ -173,16 +173,6 @@ pr-validate: lint typecheck test-unit test-acceptance-sqlite
 ci-postgres: lint typecheck test-unit test-acceptance-postgres docker-build
     @echo "CI Pipeline with PostgreSQL completed successfully!"
 
-# --- CI Image ---
-
-# Build CI runner image locally
-ci-build:
-    @./scripts/build-ci-image.sh
-
-# Build CI runner image with custom tag
-ci-build-tag TAG:
-    @./scripts/build-ci-image.sh {{TAG}}
-
 # --- Utilities ---
 
 # Show help information
@@ -256,6 +246,8 @@ workflow-status:
     @echo "PR Workflow: Runs unit + acceptance tests on pull requests"
     @echo "Test Workflow: Runs unit tests on all branch pushes"
     @echo "Release Workflow: Runs on main branch pushes (version bump + Docker build)"
+    @echo ""
+    @echo "Note: CI image approach deprecated due to Gitea Actions workspace mounting"
 
 # Check current database provider
 db-status: