[Feature Request] Detect Fly.io in builtin platform detection #1142

Description

Detect Fly.io in the builtin platform detection (@varlock/ci-env-info), so VARLOCK_PLATFORM is set on Fly Machines and VARLOCK_ENV does not fall back to development there.

Motivation

Fly.io is a common host for long-running apps, and varlock run -- <app> is the documented way to run one there. Today nothing detects it (there is no Fly entry in packages/ci-env-info/src/platforms.ts), so on a Fly Machine VARLOCK_ENV takes the local fallback and resolves to development. A schema using @currentEnv=$VARLOCK_ENV, or deriving its own flag from it as the builtin variables docs suggest, then loads development config in production unless the flag is set explicitly in the image or fly.toml. Nothing warns about it.

Proposed Solution

Add a platform entry for Fly, next to Railway and Render:

• detect: FLY_APP_NAME together with FLY_MACHINE_ID (both set on every Machine, see runtime environment).
• name: Fly.io, so VARLOCK_PLATFORM reports it; not a CI platform, so VARLOCK_IS_CI stays false.
• VARLOCK_ENV: Fly has no environment concept of its own, so either map a detected Machine to production, or leave it unset and have varlock warn when a known hosting platform is detected with no environment mapping, instead of silently using development.
• Other fields where Fly provides them, e.g. FLY_IMAGE_REF for a build or deploy identifier.

Alternatives

• Set the environment flag explicitly in the Dockerfile or fly.toml (what we do now). It works, but the silent development fallback is easy to miss.
• A general warning when VARLOCK_ENV falls back to development inside a container with no platform detected.

Additional Information

Found while deploying a Bun app to Fly under varlock run with a flag derived from $VARLOCK_ENV.