feat(cli): run a managed background preview in every launch mode

[miguel-heygen]

--background was rejected outside the embedded preview server. It now re-execs the CLI in foreground, which makes it mode-agnostic by construction: whichever server the child resolves to serves the config endpoint the readiness probe looks for.

--foreground is its counterpart, for a non-interactive shell that wants to stay attached. A bare launch keeps the same promise it always had — attached in an interactive terminal, managed in an agent session.

An existing hole this exposed

Local-studio mode runs Vite with the studio package as its working directory and needs that package's own Vite config, which the published tarball does not carry. Resolving the package was treated as proof the mode was usable, so a project with an npm-installed studio took a path that can never come up — previously a clear error, now a ten-second silent timeout once background routes through it.

The predicate becomes "can this studio actually be served", so a published install falls back to embedded mode, which works. Publishing the studio's Vite config is a packaging change worth doing separately.

Over the line budget

~1.2k changed lines against the convention's 1k. The overage is one command file and its tests carrying one invariant, and the seam that would split it further runs through a single request-handling function — a split there produces two PRs neither of which starts a preview on its own. Flagging rather than hiding it.

Verifying

Start with --background --json and confirm ready: true, then --status, then --stop, and confirm the port is free and the session record removed. Conflicting lifecycle flags produce one failure envelope.

Stack

Last of four on the preview side, based on u5-preview-json-contract. Retarget to main before merging.