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

[anthonykim1]

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: microsoft/vscode#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:

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

Add the env map from the original proposal:

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

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)