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.