Agent-host: Spawn from CLI and connect to remotes from Sessions

[connor4312]

Exposes --agent-host-port/--agent-host-path to specify an additional port for them to listen on. This runs as a separate process mirroring the architecture from desktop.

Does a 'hello world'.

There are changes to the import logic. I honestly don't know a lot about the import changes Opus is doing here . Without them, there is an error:

import { CopilotClient } from "@github/copilot-sdk";
         ^^^^^^^^^^^^^
SyntaxError: The requested module '@github/copilot-sdk' does not provide an export named 'CopilotClient'

This was Copilot's explanation for why they're needed here but not in Electron:

The agent host process itself didn't change — the issue is the module resolution environment differs between Electron and the server.

Electron (desktop): ElectronAgentHostStarter spawns a utility process that inherits the main process's module resolution. It resolves packages from the root node_modules, where @github/copilot-sdk, ws, etc. are all installed. Everything just works.

Server: NodeAgentHostStarter spawns a child via bootstrap-fork.js, which registers a custom ESM resolver hook (bootstrap-import.ts) that redirects all module lookups to node_modules instead of root node_modules. That's the [bootstrap-import] Initialized node_modules redirector for: ...\remote\node_modules log line. The remote folder has its own package.json with a curated set of server-specific dependencies.

The problems were:

1. Missing deps — @github/copilot-sdk, @github/copilot, and ws weren't in package.json because the agent host had never run in the server context before
2. Resolver bugs — bootstrap-import.ts was written when all remote deps were CJS. It hardcoded format: 'commonjs', didn't handle exports maps, didn't handle .mjs files, and didn't resolve subpath imports like vscode-jsonrpc/node. These are pre-existing limitations that never mattered until now because no server component previously depended on ESM-only npm packages.

So in short: the agent host code is identical — it's the server's module resolution plumbing that needed updating to support the ESM packages the agent host depends on.

cc @bpasero as the expert in this area