feat(cli): extend run command with port, attach, session-id, on-complete, and json options

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

src/cli/run/json-output.ts

@@ -0,0 +1,42 @@
+import type { RunResult } from "./types"
+
+export interface JsonOutputManager {
+  redirectToStderr: () => void
+  restore: () => void
+  emitResult: (result: RunResult) => void
+}
+
+interface JsonOutputManagerOptions {
+  stdout?: NodeJS.WriteStream
+  stderr?: NodeJS.WriteStream
+}
+
+export function createJsonOutputManager(
+  options: JsonOutputManagerOptions = {}
+): JsonOutputManager {
+  const stdout = options.stdout ?? process.stdout
+  const stderr = options.stderr ?? process.stderr
+
+  const originalWrite = stdout.write.bind(stdout)
+
+  function redirectToStderr(): void {
+    stdout.write = function (chunk: string): boolean {
+      return stderr.write(chunk)
+    }
+  }
+
+  function restore(): void {
+    stdout.write = originalWrite
+  }
+
+  function emitResult(result: RunResult): void {
+    restore()
+    originalWrite(JSON.stringify(result) + "\n")
+  }
+
+  return {
+    redirectToStderr,
+    restore,
+    emitResult,
+  }
+}