`copilot --acp` does not work with `--agent`

### Describe the bug

Hello! First let me thank you for your hard work. Here is below my described issue. I used copilot a bit to help on having a proper reproducer command and a potential hint. Thanks a lot guys!

---

I created a custom agent in `~/.copilot/agents/test.agent.md`
```
---
name: test
description: A minimal test custom agent
tools: []
---
You are the test custom agent. Always respond by saying: "Hello from test custom agent!"
```

When I launch copilot in "human mode" with `copilot --agent test` it works.
```
● Selected custom agent: test
▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄
❯ hello
▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
● Hello from test custom agent!
```

However when I try to use use ACP, with `copilot --acp --agent test`. It ALWAYS use the default Copilot agent. I can only use my custom agent as subagent, for example by saying "use test agent for <...>".

It seems like when using `--acp`, the `--agent` option is totally ignored.

### Affected version

GitHub Copilot CLI 1.0.65

### Steps to reproduce the behavior

## How to reproduce

#### 1. Create a minimal custom agent file in ~/.copilot/agents/test.agent.md:
```
---
name: test
description: A minimal test custom agent
tools: []
---
You are the test custom agent. Always respond by saying: "Hello from test custom agent!"
```

#### 2. Run this single command in your terminal to initialize a session and print the active agent configuration:
```
(echo '{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":1,"clientCapabilities":{},"clientInfo":{"name":"test"}}}'; echo '{"jsonrpc":"2.0","id":1,"method":"session/new","params":{"cwd":"'"$(pwd)"'","mcpServers":[]}}'; sleep 6) | copilot --acp --agent test --yolo | grep -o '"id":"agent"[^}]*"currentValue":"[^"]*"'
```

## Expected behaviour
The active agent configuration option should reflect the command-line override ("test"):
```
"id":"agent","name":"Agent","currentValue":"test"
```

## Actual behaviour
The active agent configuration is empty, falling back to standard Copilot:
```
"id":"agent","name":"Agent","currentValue":""
```

### Expected behavior

Copilot should be launched in ACP with my custom agent as main agent. I want my session to have only the tools and context I have defined in my subagent, not with the general copilot gent. I don't want my custom agent to be available just as a tool.

## Expected behaviour
The active agent configuration option should reflect the command-line override ("test"):
```
"id":"agent","name":"Agent","currentValue":"test"
```

### Additional context

- Operating System: Linux (Kernel v24.16.0 or similar)
- CPU Architecture: x86_64 / x64 (installed as @github/copilot-linux-x64)
- Terminal Emulator: Ghostty (term/ghostty)
- Shell: Bash (v5.x)
- Copilot Version: v1.0.65
[copilot-acp-agent-bug.log](https://github.com/user-attachments/files/29375552/copilot-acp-agent-bug.log)

---

## Analysis by Copilot himself

- When starting in `--acp` server mode, the CLI action handler calls `startServerMode({...}, authManager)`. However, the parsed value of `--agent` (`t.agent` / `agentName`) is completely omitted from the configuration object passed to `startServerMode`.
- There is no way for the ACP client to specify the agent in the `session/new` params, and the ACP server does not implement the `session/set_config_option` method, making it impossible to change the active agent after session creation.

An analysis of the bundled index.js reveals exactly where the flow breaks:

#### 1. CLI Flag Parser:
The parser registers the --agent option correctly:
.option("--agent ", "Specify a custom agent to use")

#### 2. Server Mode Activation Block:
When `--acp` is supplied, the CLI invokes `startServerMode` like so:
```js
let { startServerMode: ee } = await Promise.resolve().then(() => (tIn(), Sto));
await ee({
port: de,
stdio: $,
logDir: G,
logLevel: b,
settings: p,
shutdownService: d,
featureFlags: r
}, a)
```
Notice that `t.agent` (the custom agent command-line argument) is completely omitted here! The starting server is never supplied with the `--agent` option context.

#### 3. ACP Protocol Limitations:
- The JSON-RPC request for `session/new` (`Ny.SESSION_CREATE`) does not include any parameter to define or activate an agent dynamically.
- While the ACP protocol schema defines a `session/set_config_option` method, the `registerMethods` routing mechanism on the server lacks an implementation of `setSessionConfigOption` (it is defined in the type definitions/client adapter `OSe` but is completely absent from the actual server class router).

### Suggestion for fix:
Pass the command-line override into `startServerMode` configuration parameters and use it to define the initial/default `currentValue` of the `"agent"` configuration option during session setup.

Start by tracing the ACP launch path in the bundled index.js from the --acp action to startServerMode, checking how the parsed --agent value is handled. Review session/new and the registerMethods routing mentioned in the report, then run the supplied reproducer and confirm that the agent configuration reports "test" rather than an empty value.

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