`opencode serve` shows "Unexpected error" with no details when startup fails

Description

When opencode serve fails to start (e.g. port conflict, missing config), the error output is:

Error: Unexpected error

ServeError

No actionable information is shown — no indication of what went wrong (port in use, missing config, permission error, etc.).

The root cause is in packages/opencode/src/index.ts lines 128-134: the catch block calls FormatError(e) which returns undefined for server startup errors (no matching _tag), then falls through to the generic "Unexpected error" + errorMessage(e). The Effect error from Server.listen likely has a structured cause that errorMessage() can't extract a useful string from — it just prints the _tag ("ServeError") with no underlying message.

OpenCode version

v1.18.29

Steps to reproduce

1. Run opencode serve --port 4096 --hostname 0.0.0.0 on a port that's already in use (or from a directory where startup fails)
2. Observe the error output — only "Unexpected error" and "ServeError" are shown
3. Expected: the actual reason for the failure (e.g. "EADDRINUSE: port 4096 already in use" or the specific config error)

Operating System

Linux (Ubuntu)