Add the onboarding surface variant to the expert components

[cstns]

Part of #8381 / #8380, stacked on #8436. First of two PRs for the onboarding surface; the page that activates all of this comes in the next one.

This teaches the shared expert chat components an expert-surface variant, injected with a 'drawer' default so nothing changes anywhere until a surface provides 'onboarding'. Under that variant:

• ExpertMessages renders through a new pure buildCollapsedTranscript helper: past question turns (the AI questions message plus the composed human reply) fold into folded-turn entries, the collapsing transcript treatment from the mockups on Build the onboarding UI #8380. The drawer path maps messages through unchanged.
• A new CollapsedQuestionTurn renders the fold: one quiet line per question with the picks as pill chips (one chip per pick for multi selects), a Skipped marker for unanswered stale turns, expand/collapse to the original messages, and chip clicks loading the "question answer" line into the composer, since correcting a sent answer means sending a new message.
• Expert.vue hides the info banner and skips the canned welcome message (the Expert opens the onboarding conversation itself, Open the onboarding conversation without waiting for the user #8369).
• ExpertChatInput hides Start over, Plan mode and the settings cog, and reads "Or just tell me in your own words".

Everything is inert without a provider, so the reviewable claim here is that the drawer rendering is byte identical. Covered by 12 unit tests over the fold helper and the folded turn component.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.88%. Comparing base (b95185a) to head (302223c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8439   +/-   ##
=======================================
  Coverage   76.88%   76.88%           
=======================================
  Files         460      460           
  Lines       24741    24741           
  Branches     6596     6596           
=======================================
  Hits        19022    19022           
  Misses       5719     5719           

Flag	Coverage Δ
backend	76.88% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.