auth: multiple credentials per environment, keyed by account name

[MingruiZhang]

Follow-up to landing-ai/ade-cli-internal#72 (fixed by landing-ai/ade-cli-internal#78), which was explicitly scoped to fixing the login/logout overwrite-and-selection confusion. That change made environments coexist but kept one credential per environment.

This issue tracks the deferred half of landing-ai/ade-cli-internal#72's title — "like env, account name": supporting multiple credentials within a single environment, addressable by account.

Motivation

Today a second login to the same environment replaces the first. Someone with two production accounts (e.g. two identities/tenants, or an API key and an OAuth session) can't keep both.

Current model (post-landing-ai/ade-cli-internal#78)

• A profile is keyed by environment only; credentials.json holds one entry per env under environments.
• login --env X authenticates/selects X; login (flagless) targets production.
• logout is per environment.

See docs/adr/0001-auth-login-selects-logout-per-env.md for the model and why account-level profiles were deferred.

Rough scope (to be grilled before building)

• Profile identity becomes (environment, account); storage grows a nested map.
• Account naming: auto-derive from OAuth token identity (email/sub); explicit --account/--name for API keys.
• Selection within an env (which account is active) + how status/logout address accounts.
• Migration of the existing one-credential-per-env schema.

Design decisions (identity model, selection UX, storage layout) should go through a /grill-with-docs session and a new ADR before implementation.

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Migrated from the private development tracker (landing-ai/ade-cli-internal#79) during the launch cutover.