Add ACP (Agent Client Protocol) support

[yazelin]

Summary

Adds native ACP (Agent Client Protocol) support to allow the Copilot SDK to connect directly to ACP-compatible agents (e.g., Gemini CLI with --experimental-acp) without requiring a protocol translation proxy.

Closes #377

Changes

• Add protocol option to CopilotClientOptions ("copilot" | "acp")
• Implement protocol adapter abstraction layer
• Add NDJSON transport for ACP wire format
• Translate Copilot SDK methods to ACP:
  • ping → initialize
  • session.create → session/new
  • session.send → session/prompt
• Map ACP session/update notifications to SessionEvent format
• Handle Gemini's stopReason response for session.idle event

Usage

import { CopilotClient } from "@github/copilot-sdk";

const client = new CopilotClient({
  cliPath: "gemini",
  cliArgs: ["--experimental-acp"],
  protocol: "acp"
});

await client.start();
const session = await client.createSession();

session.on((event) => {
  if (event.type === "assistant.message_delta") {
    process.stdout.write(event.data.deltaContent);
  }
});

await session.send({ prompt: "Hello!" });
await client.stop();

Test Plan

• Unit tests for ACP transport (17 tests)
• Unit tests for ACP mapper (15 tests)
• Unit tests for ACP adapter (11 tests)
• E2E tests with Gemini CLI (12 tests, 10 skip without CLI)
• Manual testing with real Gemini CLI

Files Changed

src/
  client.ts                      # Modified - protocol adapter integration
  types.ts                       # Modified - added protocol option
  protocols/
    protocol-adapter.ts          # New - adapter interface
    acp/
      acp-adapter.ts             # New - ACP protocol adapter
      acp-transport.ts           # New - NDJSON transport
      acp-types.ts               # New - ACP message types
      acp-mapper.ts              # New - format translation

[yazelin]

Update: Tool Calls & Permission Request Support Added

This PR now includes complete ACP support with:

New Features

• Tool Calls: Map ACP tool_call → tool.execution_start, tool_call_update → tool.execution_complete/progress
• Permission Requests: Handle ACP session/request_permission server-to-client requests

Test Results (All Passing)

• Unit tests: 53 tests (17 transport + 21 mapper + 15 adapter)
• E2E tests with Gemini CLI: 14 tests
  • ✅ Connection and ping
  • ✅ Session creation and messaging
  • ✅ Streaming via assistant.message_delta
  • ✅ Tool events (tool.execution_start, tool.execution_complete)
  • ✅ Error handling for unsupported methods

E2E Tool Call Test Output

Tool started: search list_directory-xxx
Tool completed: list_directory-xxx success
Received 2 tool events:
  - tool.execution_start: {"toolCallId":"read_file-xxx","toolName":"read"}
  - tool.execution_complete: {"toolCallId":"read_file-xxx","success":true}

Ready for review! 🚀