When port appears available but binding fails (race with another opencode
instance), retry on next available port (auto mode) or attach to existing
server (explicit port mode) instead of crashing with exit code 1.
Implement all 5 CLI extension options for external orchestration:
- --port <port>: Start server on port, or attach if port occupied
- --attach <url>: Connect to existing opencode server
- --session-id <id>: Resume existing session instead of creating new
- --on-complete <command>: Execute shell command with env vars on completion
- --json: Output structured RunResult JSON to stdout
Refactor runner.ts into focused modules:
- agent-resolver.ts: Agent resolution logic
- server-connection.ts: Server connection management
- session-resolver.ts: Session create/resume with retry
- json-output.ts: Stdout redirect + JSON emission
- on-complete-hook.ts: Shell command execution with env vars
Fixes#1586
