Add per-session env overrides for built-in shell tool child processes

Follow up to #1911. The `initScripts` half of that ask shipped through the runtime in github/copilot-agent-runtime#12710, but the per-session `env` part was deferred out of that PR.

Context: https://github.com/microsoft/vscode/issues/323164

`initScripts` covers the Python activation case since the host can materialize a shell-specific activation file and have the runtime source it before each command. An explicit env map is still worth having for hosts that just want to set or unset a few variables without generating script files, and without dealing with per-shell quoting.

Current wire shape:

```ts
export interface ShellOptions {
initProfile?: "none" | "non-interactive";
initScripts?: ShellInitScript[];
processFlags?: string[];
}
```

Add the `env` map from the original proposal:

```ts
export interface ShellOptions {
/** Env applied to built-in shell tool child processes. `null` unsets/removes. */
env?: Record;
}
```

The reason this was deferred is that the update semantics should be pinned down before the wire shape ships:

- Whether `session.rpc.options.update` with `shell.env` replaces the whole overlay or merges per key. Replace seems right: it stays idempotent and a host can revert a variable to the inherited value by omitting it. With merge there is no way to un-touch a variable once set.
- Precedence: overlay applied to the spawned process env first, `initScripts` sourced after, so scripts can override.
- `null` meaning remove-from-env vs revert-to-inherited.
- Whether the overlay persists across resume or the host re-sends it, matching how the rest of `shell` behaves today.

(generated by copilot)

首先跟踪 issue 中描述的 ShellOptions wire 结构以及 session.rpc.options.update 的处理方式。检查 shell 和 initScripts 在更新和恢复过程中如何运行，然后明确替换、优先级、通过 null 删除以及持久化的语义；当 env overlay 能够一致地表示并应用于内置 shell 的子进程时，即视为完成。

由索引模型根据 Issue 内容生成。