Shell tool environment does not inherit user PATH - Nix-managed binaries unavailable in sessions

## Describe the bug

In worktree-backed sessions, the bash tool runs with a minimal PATH that does not include paths configured via `launchd.user.envVariables` or `/etc/profiles/per-user/$USER/bin`.

On macOS with Nix (nix-darwin + home-manager), binaries like `gh` are installed at `/etc/profiles/per-user/daniel/bin/gh` but are not reachable inside sessions. The shell only sees:

```
/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin
```

The `environment_context` block in sessions also documents this limitation explicitly:

```
Available tools: git, curl
```

There is currently no way to extend or configure additional paths for the session shell.

## Affected version

## Steps to reproduce the behavior

1. Install `gh` via Nix on macOS (nix-darwin + home-manager), binary lands at `/etc/profiles/per-user/$USER/bin/gh`
2. Open a Copilot CLI worktree session
3. Run `which gh` or `gh --version` in the bash tool
4. Command fails with `command not found`

## Expected behavior

The session shell should inherit the user's launchd environment PATH (set via `launchd.user.envVariables`), or provide a way to configure additional PATH entries (e.g. via `.copilot/config.json`).

## Additional context

- macOS aarch64 (Apple Silicon)
- Package manager: nix-darwin + home-manager
- `gh` present at `/etc/profiles/per-user/daniel/bin/gh`
- PATH is correctly set for all other GUI apps and terminals after `launchd.user.envVariables` is only Copilot CLI sessions are affectedconfigured
- Workaround: call binaries via absolute path (fragile, not documented)

未指定源文件或测试。首先在 macOS 上复现基于 worktree 的会话，并将其 bash PATH 与 launchd.user.envVariables 和 /etc/profiles/per-user/$USER/bin 进行比较；当该会话能够访问已配置的用户二进制文件（例如 gh），或支持有文档说明的额外 PATH 配置时，即表示完成。

由索引模型根据 Issue 内容生成。