feat(cli): preview --detach — a background server that manages its own lifecycle

[WaterrrForever]

What

preview --detach starts the embedded studio as a detached child, waits for it to report ready, prints the URL plus a ready-to-hand project link, opens the browser, and returns — the command exiting no longer takes the server with it. The server then watches its own lifecycle instead of a terminal: an idle timeout, plus an optional owner pid. preview --stop stops this project's server (per-project counterpart to the existing --kill-all).

Changes

• server/detachedPreview.ts (new) — the parent↔child handshake and the lifecycle policy:
  • State file .hyperframes/preview/server.json keyed by a per-launch nonce, log at server.log — a stale file from a previous launch is never mistaken for this one.
  • waitForServerState re-reads once after noticing the child exited, so the reuse path (write state, exit immediately) isn't misread as a startup failure.
  • detachedShutdownReason (pure) — exit when the explicit --owner-pid is gone, or after an idle hour (HYPERFRAMES_PREVIEW_IDLE_TIMEOUT_MS to tune, 0 disables). Deliberately no process-tree guessing for a default owner: the pid chain through npx varies by harness, and a wrong guess kills the server under the user.
• commands/preview.ts:
  • --detach (parent): spawn a detached child (--detached-child, internal), wait on the state file, print the summary + Open link (--json for the machine shape), open the browser, exit. On failure, print the log tail instead of a bare error.
  • Detached child: reports through the state file in both branches — fresh server, or already-running reuse (then exits; the existing server's lifecycle stays with whoever started it). Request activity feeds the idle clock; a 30 s watcher SIGTERMs into the existing graceful-shutdown path, which also removes the state file.
  • --stop: SIGTERM the server(s) whose projectDir matches this project.
• Docs — hyperframes-cli preview-render.md: the new flags + "From an agent, prefer --detach" guidance; skills-manifest regenerated.

Guardrails

• Nothing changes unless --detach is passed — plain preview keeps today's foreground behavior exactly; under the Vite dev/local-studio modes --detach warns and runs foreground.
• The idle/owner watcher logs its reason before SIGTERM-ing itself.
• clearServerState only deletes its own launch's file — an older child shutting down can never clobber a newer server's state.

Tests / verification

• 1445/1445 cli tests green (16 new: state-file round-trip, corrupt-file tolerance, the write-then-exit race, stale-launch rejection, the shutdown-reason matrix, idle-limit env parsing).
• E2E smoke on the built CLI: --detach --json returns once serving with the full state payload; the API answers; --stop SIGTERMs it and graceful shutdown removes the state file.
• typecheck, oxlint, oxfmt clean.