Change OpenCode startup behavior

[crankshift]

Summary

Change opencode-remote run startup behavior so OpenCode is not silently auto-started by default when the API is unavailable.

Context

Startup should be explicit and predictable. Interactive users should be prompted before starting opencode serve, while non-interactive environments need an explicit flag or environment option for auto-start behavior.

Acceptance Criteria

• When opencode-remote run starts and OpenCode is unavailable, interactive CLI users are prompted before starting opencode serve.
• OpenCode is not silently auto-started by default.
• A non-interactive flag or environment option exists for explicit auto-start behavior.
• Runtime ownership rules remain intact: the gateway stops only the OpenCode child process it started.
• README and relevant development docs describe the new startup behavior.
• Tests cover interactive prompting, default no-silent-autostart behavior, and explicit non-interactive auto-start behavior.

Notes

Keep user-facing errors safe and avoid exposing local paths, stack traces, or provider details.