Migrate the token action from app-id to client-id #93

actions/create-github-app-token now warns on every run:

Input 'app-id' has been deprecated with message: Use 'client-id' instead.

This is not a rename. The two inputs take different values.

• SIMPLYCUBED_GH_APP_ID holds the App ID, a number (for this App, 4425872).
• client-id expects the App's Client ID, an Iv23… string shown on the same settings page.

Swapping the input name while the stored variable still holds the App ID would break token minting, and with it every run, to silence a warning. I started that change during the v0.1.7 prep and backed it out for exactly that reason.

Work

1. Add a repository variable SIMPLYCUBED_GH_APP_CLIENT_ID holding the Client ID. Keep the existing App ID variable until the migration is proven, so rollback is one line.
2. Switch the token step in .github/workflows/simplycubed.yml, .github/workflows/tag-release.yml, and the self-test template to client-id, reading the new variable.
3. Update init's printed next steps, the README install section, and docs/setup.md, which all currently name only the App ID variable.
4. Prove it with the self-test before removing the old variable. Its first step fails when the credentials are wrong, so a bad migration is caught in a minute rather than on the next real run.

Acceptance

• No deprecation warning in the run log.
• The self-test passes with only the Client ID variable set.
• An adopter following the docs from scratch sets the right variable and nothing else.