docs(cli-generator): document built-in auth command merging

[devin-ai-integration]

Summary

End users of a generated CLI can now log in through the browser (authorization code + PKCE) or a device code without a client secret, and APIs that define their own auth operations no longer lose them to the framework's built-in auth login / logout / status. This updates the CLI generator authentication page with the command-merging behavior and its collision rule, and corrects the minimum generator version for the public-client OAuth flows.

Implements docs for: feat(cli): wire OAuth public-client login flows (PKCE + device-code) (fern-api/fern#17277) and fix(cli-generator): fold built-in auth commands into a spec-owned auth group (fern-api/fern#17286)

The OAuth flow configuration itself (flow types, redirect-uri, extra parameter maps, login UX) was already documented in #6475, so this PR only fills the gaps:

• authentication.mdx — built-in login / logout / status (and completion / man) fold into a spec-defined group of the same name, so my-cli auth me and my-cli auth login both dispatch; exact leaf collisions resolve in favor of the built-in, with x-fern-sdk-method-name as the workaround. Also notes that PKCE always applies to authorization-code with S256 as the only challenge method, and fixes the stated minimum generator version (0.29.0 → 0.30.0, per generators/cli/versions.yml).
• openapi-extensions.mdx — one sentence under x-fern-sdk-method-name pointing at the collision workaround.

Vale and fern check --warnings pass (remaining warnings are preexisting).

Link to Devin session: https://app.devin.ai/sessions/0aab9a94686d48c38b86733540659c5f

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1785517716-cli-generator-auth-commands.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

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