feat(host): manage multiple Ellipsis instances (agent host)

[hbrooks]

What

Adds an agent host command group so the CLI can target multiple Ellipsis instances — Ellipsis Cloud, a preview environment, or a self-hosted deployment — and switch between them without re-exporting env vars.

agent host add beta https://beta-api.ellipsis.dev   # register + switch to it
agent host add prod https://api.ellipsis.dev
agent host list                                      # active marked *
agent host use beta
agent host current                                   # api + app + how it resolves
agent host set beta --rename staging --app-base https://dash…
agent host delete beta

agent host sets where the CLI points; agent login sets the credential for wherever it's pointing. No overlap, no --login shortcut.

Model

• Config becomes { version: 2, activeHost, hosts: { <name>: { apiBase, appBase?, token?, enrolledRepos? } } }.
• Token and enrolled-repos are per host — switching instances never reuses another instance's credential or sync consent.
• App/dashboard URL is per host: derived from the API URL (api. → app.) by default, or set explicitly with --app-base for a self-hosted instance whose dashboard host isn't a mechanical swap. Every dashboard link the CLI prints (including the login verification URL) uses it.
• Precedence unchanged: explicit arg → ELLIPSIS_API_BASE_URL/ELLIPSIS_API_TOKEN env → active host → prod default. Sandbox env injection still works untouched.
• A pre-hosts (v1) config is migrated in place on first load — the existing login becomes a host named for its API base — so nobody has to re-login.

Login fix (supersedes #36)

agent login now builds the approval URL from the active host's app base instead of the server's verification_uri_complete. The backend fills that from ELLIPSIS_APP_BASE_URL, which is unset in every env and defaults to prod — so a beta/self-hosted login was sent to the prod dashboard, where the code can't be approved (login hung). Closes #36.

The backend is still wrong for non-prod (separate consolidation: set ELLIPSIS_APP_BASE_URL per instance + unify the app-host helpers); the CLI no longer depends on it. Server-authoritative app-URL learning is a clean future enhancement once that lands.

Test plan

• bun run typecheck, bun run test (202 pass — new host/migration/cliAuthUrl cases), bun run build, compile smoke — all green locally.
• Manual: add/use/list/current/set/delete verified end to end; --app-base override and v1→v2 migration confirmed.