feat(server): wire Server.listen() through native HttpApi listener (kill-switch)

When the effect-httpapi backend is selected, Server.listen() now delegates
to HttpApiListener.listen() — a native Bun.serve listener with inline
WebSocket upgrade handling — instead of routing through the Hono runtime
adapter (Hono.fetch + createBunWebSocket).

The Hono backend path is unchanged, and a kill-switch env var
(OPENCODE_HTTPAPI_LEGACY_LISTENER) forces the effect-httpapi backend back
through the Hono adapter as an escape hatch if the native listener
regresses for a user.

This unblocks the Hono deletion arc by giving the native listener real
production traffic on dev/beta/local channels (where
OPENCODE_EXPERIMENTAL_HTTPAPI defaults on) while leaving prod/latest
channels on the Hono path.