CI: Use custom container for E2E tests (#7625)

## Summary
Use a pre-built container image with all dependencies for Playwright E2E
tests, eliminating ~130s setup time per shard.

## Changes
- Use `ghcr.io/comfy-org/comfyui-ci-container:0.0.8` container for test
jobs
- Container includes: Playwright browsers, Node.js, pnpm, Python,
ComfyUI backend (v0.5.1), all Python deps
- Simplified setup: just copy devtools and start server (no cloning, no
pip install, no browser setup)
- Add `version-bump*` to `branches-ignore` to skip E2E tests for version
bump PRs
- Replace cache with artifacts (cache doesn't work inside containers)

## Benefits
- ~130s faster per shard (no ComfyUI clone, no pip install, no browser
download)
- Consistent environment across all test jobs
- Simpler workflow configuration

## Container Image
Repository: https://github.com/comfy-org/comfyui-ci-container
Image: `ghcr.io/comfy-org/comfyui-ci-container:0.0.8`

## Test plan
- [x] Verify CI workflow runs with container
- [x] Verify Playwright tests pass
- [x] Verify snapshot updates work correctly

---------

Co-authored-by: github-actions <github-actions@github.com>

.github/actions/start-comfyui-server/action.yml

@@ -0,0 +1,23 @@
+name: Start ComfyUI Server
+description: 'Start ComfyUI server in a container environment (assumes ComfyUI is pre-installed)'
+
+inputs:
+  front_end_root:
+    description: 'Path to frontend dist directory'
+    required: false
+    default: '$GITHUB_WORKSPACE/dist'
+  timeout:
+    description: 'Timeout in seconds for server startup'
+    required: false
+    default: '600'
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Copy devtools and start server
+      shell: bash
+      run: |
+        set -euo pipefail
+        cp -r ./tools/devtools/* /ComfyUI/custom_nodes/ComfyUI_devtools/
+        cd /ComfyUI && python3 main.py --cpu --multi-user --front-end-root "${{ inputs.front_end_root }}" &
+        wait-for-it --service 127.0.0.1:8188 -t ${{ inputs.timeout }}