feat(ext): first-time setup walkthrough on the options page

[paperhurts]

Summary

Onboarding polish from epic #61 Phase 1: an in-extension setup walkthrough with a "create your bookmarks repo" deep link and clearer PAT guidance on the options page.

Changes

• First-time setup guide (options.html): a <details> block above the form with three steps —
  1. Create a private repo (pre-filled via query params), with a note that any existing repo works too;
  2. Create a fine-grained token with the exact steps (Only select repositories → the bookmarks repo; Contents: Read and write; nothing else) and what to do at expiry;
  3. paste, Validate, Save.
• Guide auto-opens when no settings exist (first run / post-corruption reset shows the form blank anyway) and collapses on successful save (options.ts).
• PAT guidance: github_pat_… placeholder + hint that the token is stored only in this browser's extension storage and sent only to api.github.com — no gitmarks server.
• Saved status now says what to do next: ✓ saved — click the gitmarks icon on any page to save it. The three options-page e2e assertions moved from toHaveText("✓ saved") to toContainText (two of them are in skipped tests, updated for consistency; the popup-save assertion is untouched).
• Dropped a pre-existing unused browser import in options.ts.

Single-source change: both shells copy options.html from extension-shared at prebuild, so Chrome and Firefox pick this up identically.

Verification

• pnpm --filter @gitmarks/extension-shared test — 127 passed
• pnpm typecheck — all packages clean
• Both shells build
• Real browser: full Chrome e2e suite — 7 passed, 2 (intentionally) skipped, including the real-GitHub reconcile round-trip
• Fresh-profile Chromium screenshot confirms the guide auto-opens and renders in the web-UI palette

Notes

• The github.com/new query-param pre-fill (name/visibility/description) is best-effort — if GitHub ignores a param the link still lands on repo creation. Worth an eyeball during the next manual smoke test.
• The store-listing options screenshot (docs/store assets) predates this change and should be re-captured before submission.

Part of #61

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