Add Playwright E2E suite against the built dist/

Pin @playwright/test 1.62.0 (matches the cached chromium
revision) and serve dist/ via python3 http.server, so the
deployed artifact is what gets tested. Covers page load,
terminal commands, vim mode, achievements, and the nav menu.
Ignore node_modules, test-results, playwright-report, and the
phased-execution runtime scratch (.agents/phase-sessions,
pipeline.log).

.gitignore

@@ -8,6 +8,16 @@ dist/
 # Local Docker build cache
 .docker/
 
+# JS / test tooling
+node_modules/
+test-results/
+playwright-report/
+
+# Phased-execution runtime artifacts (the .agents/ planning tree itself is
+# tracked and committed — only its per-run scratch is ignored)
+.agents/phase-sessions/
+.agents/pipeline.log
+
 # OS files
 .DS_Store
 Thumbs.db