Agentic UI: Add a first-run orientation guide

[bcotrim]

Related issues

• Related to STU-2016 (second slice of the Workbench Orientation work; the first was Agentic UI: Add suggested starter prompts over the empty chat #4225)
• Prior art: exploration in Agentic UI: Design Preview #3975

How AI was used in this PR

Built with Claude Code: ported the orientation-guide exploration from the #3975 branch onto trunk, re-verified every integration point against current trunk, trimmed the scope to just the guide, and verified with lint, typecheck, and unit tests. Manually reviewed code and tested.

Proposed Changes

This adds a short orientation guide that opens once on first arrival.

Anyone can replay it later from Help ▸ Getting Started, following the same pattern What's New already uses.

Testing Instructions

1. Enable the Agentic UI beta feature.
2. To simulate a first run, remove onboardingHints from ~/.studio/app.json (or set tourCompletedVersion/tourDismissedVersion to 0) and relaunch.
3. With at least one site, arrive in the workbench — the guide should open by itself about half a second later.
4. Page through it: Back appears from page two, the dots stay centered, and the last button closes it.
5. Relaunch — it should not reappear.
6. Repeat, but close with Esc or the ✕ Skip button; it should also not reappear.
7. Choose Help ▸ Getting Started — the guide reopens. Finishing or skipping the replay keeps it from auto-opening again.
8. Sign out (or go offline) and repeat step 3 — the middle page should be the site-overview variant instead of the chat one.
9. Verify in both light and dark mode.

Note: the illustration area is an intentional tinted placeholder — the real art is still to come.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?