feat: support PRISMIC_TOKEN env var to override stored token

[angeloashmore]

Resolves:

Description

Adds a PRISMIC_TOKEN environment variable that takes priority over the token stored in the credentials file. Useful for CI, scripted use, and ephemeral environments where persisting a token to disk is undesirable.

The env value is kept in memory only — refreshToken() early-returns when PRISMIC_TOKEN is set so the value is never written to the credentials file.

Mirrors the existing PRISMIC_HOST pattern in src/auth.ts.

Checklist

• A comprehensive Linear ticket, providing sufficient context and details to facilitate the review of the PR, is linked to the PR.
• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

How to QA

1. node --run build
2. With no credentials file, run PRISMIC_TOKEN=<valid-token> ./dist/index.mjs whoami — should print the email associated with that token.
3. With PRISMIC_TOKEN set, run any authenticated command and confirm the credentials file's token field is unchanged afterward.

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Note

Medium Risk
Changes how all authenticated commands resolve tokens and refresh behavior; mistakes could affect CI/scripts or leave users stuck on bad env tokens, but scope is small and mirrors PRISMIC_HOST.

Overview
Adds optional PRISMIC_TOKEN env support so authentication can use an in-memory token instead of the on-disk credentials file—aligned with the existing PRISMIC_HOST override pattern.

getToken() returns the env value when set; refreshToken() no-ops in that case so refreshed tokens are never persisted to the credentials file. prismic init surfaces a clear error when PRISMIC_TOKEN is set but invalid, instead of falling through to browser login. Coverage includes a whoami test that runs with PRISMIC_TOKEN after logout.

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