Auto-resolve GitHub App installation_id from org/repo/user name

[STRRL]

Problem

The config requires a hard-coded installation_id for every org and repo entry. It is error-prone and awkward to obtain, especially for repo-scoped entries where the user has to open the GitHub App settings page and read the id out of the URL. The org/repo/user name is already present in the config, so the installation_id is redundant.

Proposal

When auth uses a GitHub App, resolve the installation id automatically from the configured name and make the installation_id field optional.

The GitHub REST API exposes the installation for a given account/repo directly (authenticated with the App JWT):

• Org: GET /orgs/{org}/installation
• Repo: GET /repos/{owner}/{repo}/installation
• User: GET /users/{username}/installation

Each returns the installation object, including its id.

Behavior

• If installation_id is set, use it as-is (backwards compatible).
• If omitted, resolve it at startup from the org/repo name via the endpoint above.
• Fail fast with a clear error if the App is not installed on that account/repo (the error should name the org/repo so it is diagnosable without source code).

This removes a manual, error-prone config step.