A private app on a public repository stays private: reading the code is not a key to the app or its previews #620

An app's access: private currently means "anyone who can read the repository" (the grant in openEnv in src/actions/apps.ts, and the proxy's sign-in hand-off). That holds while the repository is private. Once the repository is public, every signed-in account can read it, so a private app and every pull preview open to every account, and signup is open. Making a repository public must not change who reaches its app.

Acceptance criteria

• [ ] A private app admits only people who can write to the repository, plus anyone the owner names explicitly (a member list, or a team when teams land). Reading the repository is not enough.
• [ ] Pull previews follow the same rule as the app they belong to.
• [ ] The terminal (src/apps/tty.ts) follows the same rule.
• [ ] Changing a repository from private to public changes no app's admissions. A test proves this: a private app on a repository that becomes public refuses a signed-in reader, and still lets in a writer.
• [ ] The visibility settings page says, before you confirm, what becomes visible and what stays private (apps, previews, run logs).

Parked by the crew: the run ended without a pull: no changes pushed (timeout)