Auth: support multiple platform environments (per-host credential profiles)

While dogfooding the CLI (1.5.0, agent-driven) against a second Clerk platform environment, we hit a wall on auth: CLERK_PLATFORM_API_URL cleanly redirects requests, but the credential store only holds one OAuth token and clerk auth login has no notion of which host a token belongs to. Logging into the alternate environment would clobber the production login, so the only safe path is minting a platform API key for the second environment and threading CLERK_PLATFORM_API_KEY through every call.

That workaround is fine for CI but makes non-production work second-class for interactive use. A per-host credential store (key tokens by platform API URL) or an explicit --profile on auth login/api/config would make switching environments first-class. Bonus: clerk doctor could report which environment the stored token belongs to.

Start by tracing the credential store and the clerk auth login, api, and config command entry points described in the issue. Clarify whether per-host credentials or explicit profiles is the intended design, then verify that separate environments can be used without overwriting each other and that the relevant commands select the correct credentials.