docs: document generated CLI .env loading and default auth env vars

[devin-ai-integration]

Summary

The CLI generator's Authentication page already covered credential sources and the device-code OAuth generators.yml config, but two things customers asked about weren't stated: how end users supply credentials via a .env file, and which environment variable a scheme reads when none is declared.

Additions to fern/products/cli-generator/authentication.mdx:

• Default environment variables subsection: a scheme with no declared env var derives one from the uppercased binary name (<BINARY>_TOKEN for bearer, <BINARY>_API_KEY for header API key, <BINARY>_USERNAME/<BINARY>_PASSWORD for basic, <BINARY>_CLIENT_ID/<BINARY>_CLIENT_SECRET for OAuth client credentials), and the note that a header API-key scheme also gets an --api-key flag that wins over its env var.
• .env files subsection: the generated CLI auto-loads a .env file from the working directory or any parent at startup; a .env value never overrides an already-set env var. Credential precedence: explicit CLI flag → environment variable → .env.
• Device-code flow: clarified client-id must be a literal public client ID and added client-id-env to the list of rejected fields (alongside redirect-uri and pkce).

No new page or section was created — these fold into the existing canonical auth page.

Checks

• pre-commit run --files fern/products/cli-generator/authentication.mdx → Vale passed, 0 errors.
• fern check --warnings → 0 errors (6 pre-existing, unrelated warnings).

Link to Devin session: https://app.devin.ai/sessions/7958e622b9224604a134e001984b4239

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1785769805-cli-generator-auth-env.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/cli-generator/get-started/authentication