Add support for customizing the OpenCode server port and hostname via
environment variables. This enables orchestration tools like Open Agent
to run multiple concurrent missions without port conflicts.
Environment variables:
- OPENCODE_SERVER_PORT: Custom port for the OpenCode server
- OPENCODE_SERVER_HOSTNAME: Custom hostname for the OpenCode server
When running oh-my-opencode in parallel (e.g., multiple missions in
Open Agent), each instance can now use a unique port to avoid conflicts
with the default port 4096.
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