Bug: Detect read-only token for deployment API.

Our [`lib/github/deployment.js`](https://github.com/FormidableLabs/formideploy/blob/master/lib/github/deployment.js) just checks the presence of a token to decide if it should attempt the deployment API. But in GH actions, there is a token, just a read-only one, so it actually fails.

Tasks:

- [ ] Detect read-only token and skip deployment actions

Read lib/github/deployment.js first and trace how token presence currently selects deployment API calls. Investigate how the GitHub Actions token’s read-only status can be identified, then verify that deployment actions are skipped for it while writable tokens still attempt deployment; no test file is named in the issue.

索引モデルが issue の本文から書いたものです。