fix(compose): stop double-wrapping healthcheck test commands

[vsilent]

Every healthcheck written the way the reference documents came out broken:

/bin/sh: 1: CMD-SHELL: not found

Docker wraps a plain string test in CMD-SHELL itself, so a string that already spells the prefix out gets wrapped twice and the container tries to execute a program named CMD-SHELL. The prefix only means anything in the list form. The generator was emitting the author's string verbatim.

Observed on a real deploy: both stackpilot services that declare a healthcheck came up unhealthy while the application itself was fine. Harmless on its own, but the state is frozen into a baked snapshot, and a stack using depends_on: condition: service_healthy would never start.

The decision now lives in one place. compose_service_sync already had healthcheck_test_value doing this correctly for the server-side path; the generator delegates to it and only renders the result as inline YAML. That also picks up a subtlety a second implementation would have missed: CMD executes argv directly, so a command containing &&, |, $ or redirection is emitted as CMD-SHELL instead. An explicit list written by the author now passes through untouched.

The reference said test: "CMD pg_isready -U postgres" and left it there. It now documents all three accepted forms, what each one runs, and why the prefix works here but not in a plain compose file.