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.

Reproduce el problema con varias extensiones instaladas y cuatro sesiones abiertas en Windows 11; después, cuenta los procesos de copilot.exe que contengan extension_bootstrap. Compara la escalabilidad observada de los procesos y la memoria con el comportamiento esperado de inicio diferido o de un extension-host compartido. Se considera terminado cuando los procesos de las extensiones ya no se multiplican innecesariamente entre sesiones y las extensiones no utilizadas no se inician de forma eager.

Escrito por el modelo de indexación a partir del texto del issue.