[Improve] Remove Enter values manually from Create GitHub App

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

Related issue

Follow-up to the simplified Slack create flow — internal UX polish, no tracked issue.

Why this PR exists

• A maintainer explicitly invited this PR in the linked issue or discussion
• I am a maintainer / this is internal Roomote work

What changed

• Removed Enter values manually from Create GitHub App in setup and settings.
• Kept the automated Create GitHub App path as the only in-UI create option (existing apps can still use deployment env vars).
• Updated setup/settings copy so it no longer advertises a manual UI path.
• Setup still shows Back to the previous wizard step when available.

How it was tested

• Targeted web vitest for StepSourceControlConfig and SourceControl (27 passed)
• Pre-push (check-types:fast, residual lint, knip)
• Browser proof of the settings Create GitHub App screen without the manual button

Screenshots

Checklist

• The PR title follows the repo convention: [Fix], [Feat], [Improve], [Refactor], [Docs], or [Chore] followed by a user-facing description
• This PR is small and scoped to one change
• pnpm lint and pnpm check-types pass locally
• I added tests or included a clear manual validation note above
• I removed secrets, tokens, private keys, and customer data from code, logs, and screenshots
• If this change should appear in the changelog, I ran pnpm changeset

[roomote-roomote]

No new code issues found. See task

• apps/web/src/components/settings/SourceControl.tsx:342-345 The disconnected GitHub message still says users can enter an existing app's credentials, but this PR hides the only settings form that allowed it whenever GitHub is unconfigured. Update this copy to advertise only the remaining creation/deployment-environment path so the screen does not promise an unavailable manual flow.

_{Reviewed 3508a8}

[roomote-roomote]

Fixed — the disconnected GitHub settings copy no longer promises entering an existing app’s credentials in the UI. It now points at Create GitHub App + deployment env vars for existing apps.