Create a single session across multiple repositories

### Feature summary

Let me start one session that spans multiple repositories at once, instead of being limited to a single repo or folder per session.

### What problem are you trying to solve?

Today a session is bound to a single project — one repository or one folder. But a lot of real work spans several repos at the same time: a frontend + backend split, a service and its shared client library, an API and its SDK, or a coordinated change rippling across microservices.

To make a cross-repo change today I have to:

- Spin up a **separate session per repo**.
- Manually re-paste the same context/prompt into each one.
- Keep the sessions mentally in sync and open a PR in each, then stitch the work back together.

Worse, the agent in any one session can only see and edit *its* repo — it has no visibility into the sibling repos, which is exactly the context a cross-repo change needs. The result is a lot of session-juggling and lost context for what is conceptually a single unit of work.

### Proposed solution

Allow a session to include **multiple repositories (and/or folders)**:

- **Multi-select in the new-session view** — pick more than one repository/folder instead of exactly one.
- **One workspace, per-repo checkouts** — provision a worktree/checkout for each selected repo under a single session workspace, each on its own branch, and point the Copilot CLI process at the combined workspace so the agent can read and edit across all of them.
- **Grouped diff + per-repo PRs** — group the included repos in the session's workspace/diff view, and support creating one PR per repo from that single session, linked together.
- **Reusable repo sets (optional)** — let me save a named "multi-repo set" so I can launch such a session in one click.
- **Generalize existing entry points** — extend the "New session in repository…" picker and the `gh://session/new` deep link to accept multiple repositories.

### Workflow impact

This helps anyone whose changes routinely cross repo boundaries — full-stack developers (frontend + backend), platform and microservice teams, SDK + service owners, and orgs that split a monorepo into many repos. It removes the per-repo session juggling and manual context copying, keeps the agent's context coherent across the whole change, and turns a multi-session, multi-PR chore into a single directed workstream.

### Additional context

This generalizes capabilities the app already has for the single-repo case:

- Sessions are already grouped by repository in the sidebar.
- "New session in repository…" already lets you pick one project repo when starting from an issue.
- `gh://session/new` already deep-links into a single repository.

This request extends those single-repo entry points and grouping to support **N repos** in one session.

Commencez par lire le flux existant pour un seul dépôt « New session in repository… » et le deep link gh://session/new. Suivez la façon dont les sessions sont regroupées et dont le workspace et la vue diff gèrent un dépôt. C’est terminé lorsqu’une session peut sélectionner plusieurs dépôts ou dossiers, coordonner leurs workspaces, afficher les modifications regroupées et prendre en charge des PRs liées pour chaque dépôt.

Rédigé par le modèle d'indexation à partir du texte de l'issue.