feat(e2e): implement working admin e2e tests with Playwright

[ralphbean]

Summary

- Merges the design spec and implementation plan from PR #156

• Implements the full admin e2e test suite using Playwright for browser automation
• Fixes numerous reliability issues discovered during implementation
• Tests pass reliably on consecutive runs (~84 seconds each)

Key Changes

E2E Test Infrastructure

• E2E_GITHUB_PASSWORD_FILE support: Reads password from a file path when E2E_GITHUB_PASSWORD env var is not set
• Cross-origin manifest fix: Fetches local manifest form via HTTP and submits from GitHub's origin to avoid SameSite=Lax cookie loss
• workflow PAT scope: Required for creating files in .github/workflows/ directories
• Retry logic for 404s: CreateOrUpdateFile retries with exponential backoff for newly created repos (GitHub eventual consistency)

Browser Automation

• Replaced WaitUntilStateNetworkidle with WaitUntilStateDomcontentloaded (GitHub keeps persistent connections)
• Added hook_attributes and redirect_url to manifest JSON for GitHub's manifest flow
• Fixed app deletion: org-scoped URL, type app name to confirm, correct button selectors
• Added debug screenshot helper for troubleshooting failures

Lock and Cleanup

• Reduced lock timeout to 2 minutes, poll interval to 30 seconds
• Added stale lock detection for non-UUID content
• Uses CreateOrUpdateFile instead of CreateFile for lock (handles auto_init README)
• Cleans up stale enrollment branches via GitHub API
• Creates test-repo if it doesn't exist

Test Flow (all 6 phases pass)

1. Cleanup: Delete stale apps, repos, branches from previous runs
2. Install: Create 4 GitHub Apps, install them, write config/workflows/secrets
3. Verify: Check all resources exist (config, workflows, secrets, variables, enrollment PR)
4. Analyze: Verify layer stack reports correct status
5. Uninstall: Delete config repo
6. Verify uninstall: Confirm cleanup

Bug Fix

• Fixed compilation error in deletePAT (wrong number of return values)

[ralphbean]

Closing in favor of #167