Extensions fork once per session instead of being shared

### Short summary

Every open session forks its own copy of all installed extensions, so memory scales with sessions x extensions.

### Affected version or release

App 1.0.26 (CLI 1.0.75), Windows 11

### Installation context

13 user extensions in `~/.copilot/extensions`, 4 sessions open

### What happened?

Each session starts a separate OS process for every installed extension, whether or not the extension is ever used. Nothing is shared between sessions and nothing is lazy.

With 13 extensions and 4 sessions open: 53 extension processes, 13.5 GB.

Almost all of that is runtime overhead, not extension code.

### Steps to reproduce

1. Install several extensions.
2. Open 4 sessions.
3. Count `copilot.exe` processes with `extension_bootstrap` in the command line.

### Expected behavior

Extensions start when first used, or one extension host is shared across sessions. Memory should not multiply by session count.

Windows 11 で複数の拡張機能をインストールし、4 つのセッションを開いた状態で問題を再現し、その後 extension_bootstrap を含む copilot.exe プロセスを数えます。観測されたプロセス数とメモリのスケーリングを、遅延起動または共有 extension-host の想定される動作と比較します。拡張機能のプロセスがセッション間で不必要に増殖せず、未使用の拡張機能が eager に起動されなくなれば完了です。

索引モデルが issue の本文から書いたものです。