Follow-up: Update VS Code extension doc URLs after registration moved to sign-in-dbt-extension

## Summary

Registration and troubleshooting content was removed from the install extension docs page in the [June 2026 docs release](https://github.com/dbt-labs/docs.getdbt.com/commit/8f13ee1c3ff71d68190f2bfffd783890a3b8f80b) and moved to:

**https://docs.getdbt.com/docs/sign-in-dbt-extension**

The VS Code extension (and related packages) still link to removed anchors on the install page. Users land on `/docs/install-dbt-extension` but do not reach registration or troubleshooting content.

## Broken URLs (extension / product repos)

| Repo | File | Current URL |
|------|------|-------------|
| `dbt-labs/vsce` | `src/data/urls.ts` | `.../install-dbt-extension#register-with-an-existing-dbt-account` |
| `dbt-labs/vsce` | `package.json` | same (registration prompt description) |
| `dbt-labs/vsce` | `src/StatusBar.ts` | `.../install-dbt-extension#register-the-extension` |
| `dbt-labs/vsce` | `src/config.ts` | `.../install-dbt-extension#troubleshooting` |
| `dbt-labs/fs` | `lsp/src/data/urls.ts`, `lsp/src/StatusBar.ts`, `lsp/src/config.ts` | same patterns |
| `dbt-labs/bakery` | `packages/sourdough/.../AuthenticationLayout.tsx` | `extensionDocsLink` → `#register-with-an-existing-dbt-account` |

## Suggested replacements

| Old anchor | New URL |
|------------|---------|
| `#register-with-an-existing-dbt-account` | `/docs/sign-in-dbt-extension?registration-path=existing-account#existing-dbt-account` |
| `#register-the-extension` | `/docs/sign-in-dbt-extension#choose-a-sign-in-path` |
| `#troubleshooting` | `/docs/sign-in-dbt-extension#troubleshooting` |
| `#choose-your-registration-path` | `/docs/sign-in-dbt-extension#choose-a-sign-in-path` |

## Notes

- Docs internal links were updated in the June release; this tracks hardcoded extension/product URLs that still point at removed install-page anchors.
- `vercel.json` redirects do not fix same-page hash links; URLs need updating at the source in extension/product repos.
- Related docs PR improves Windows Fusion CLI install clarity (OS tabs); does not fix these stale extension links.

## Test plan

- [ ] Extension status bar **Help** link → sign-in path section
- [ ] Registration prompt “these steps” link → existing account sign-in tab
- [ ] Troubleshooting install link → sign-in troubleshooting section