feat(cli): implement visual validation framework and TTY smoke tests

This change introduces a multi-layered validation strategy for the Gemini CLI UI, including:
- TTY Bootstrap Smoke Tests using node-pty to validate real terminal startup.
- Visual Regression Testing using SVG snapshots and AppRig.
- Core fixes for a scheduler hang and suppressed policy violations.
- Comprehensive documentation for maintainers.

docs/sidebar.json

@@ -173,6 +173,10 @@
         "items": [
           { "label": "Contribution guide", "slug": "docs/contributing" },
           { "label": "Integration testing", "slug": "docs/integration-tests" },
+          {
+            "label": "Visual validation and TTY",
+            "slug": "docs/cli/visual-validation"
+          },
           {
             "label": "Issue and PR automation",
             "slug": "docs/issue-and-pr-automation"