Docs: a restartable private server on a fixed endpoint #509

The launcher picks port 0 and its own password, so restarting on the same endpoint needs an app-owned opencode serve. Its readiness recipe is undocumented.

Proposed: one recipe for an app-owned serve with its readiness check. An SDK option for a fixed endpoint would be an additive feature for 1.1.

Source: internal developer-experience evaluation of the 2.0.32 nightly, 2026-10-18 (finding D32-11).