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/server-connection.ts

@@ -0,0 +1,47 @@
+import { createOpencode, createOpencodeClient } from "@opencode-ai/sdk"
+import pc from "picocolors"
+import type { ServerConnection } from "./types"
+import { getAvailableServerPort, isPortAvailable, DEFAULT_SERVER_PORT } from "../../shared/port-utils"
+
+export async function createServerConnection(options: {
+  port?: number
+  attach?: string
+  signal: AbortSignal
+}): Promise<ServerConnection> {
+  const { port, attach, signal } = options
+
+  if (attach !== undefined) {
+    console.log(pc.dim("Attaching to existing server at"), pc.cyan(attach))
+    const client = createOpencodeClient({ baseUrl: attach })
+    return { client, cleanup: () => {} }
+  }
+
+  if (port !== undefined) {
+    if (port < 1 || port > 65535) {
+      throw new Error("Port must be between 1 and 65535")
+    }
+
+    const available = await isPortAvailable(port, "127.0.0.1")
+
+    if (available) {
+      console.log(pc.dim("Starting server on port"), pc.cyan(port.toString()))
+      const { client, server } = await createOpencode({ signal, port, hostname: "127.0.0.1" })
+      console.log(pc.dim("Server listening at"), pc.cyan(server.url))
+      return { client, cleanup: () => server.close() }
+    }
+
+    console.log(pc.dim("Port"), pc.cyan(port.toString()), pc.dim("is occupied, attaching to existing server"))
+    const client = createOpencodeClient({ baseUrl: `http://127.0.0.1:${port}` })
+    return { client, cleanup: () => {} }
+  }
+
+  const { port: selectedPort, wasAutoSelected } = await getAvailableServerPort(DEFAULT_SERVER_PORT, "127.0.0.1")
+  if (wasAutoSelected) {
+    console.log(pc.dim("Auto-selected port"), pc.cyan(selectedPort.toString()))
+  } else {
+    console.log(pc.dim("Starting server on port"), pc.cyan(selectedPort.toString()))
+  }
+  const { client, server } = await createOpencode({ signal, port: selectedPort, hostname: "127.0.0.1" })
+  console.log(pc.dim("Server listening at"), pc.cyan(server.url))
+  return { client, cleanup: () => server.close() }
+}