Add an app-wide default for session workspace type

### Feature summary

Add an app-wide setting that controls whether new local project sessions start in a new worktree or use the repository's existing workspace.

### What problem are you trying to solve?

The GitHub Copilot app currently defaults new local project sessions to isolated worktrees. Some workflows require every session to operate in the repository's existing workspace instead, but `workspace_type: "branch"` must currently be selected for each session individually. This is easy to forget and cannot be made consistent across newly created sessions.

### Proposed solution

Add an app-level preference for the default workspace type used when creating local project sessions:

- **New worktree** (current behavior and backward-compatible default)
- **Existing workspace** (`workspace_type: "branch"`)

Apply the preference to sessions created from the UI, chat, issues, pull requests, and scheduled workflows whenever no explicit workspace type is supplied. Explicit per-session choices should override the preference. If using the existing workspace could conflict with another active session, show the existing warning or confirmation rather than silently creating a worktree.

### Workflow impact

Developers who intentionally use one checked-out workspace—for example because of local environment configuration, large repositories, limited disk space, or tools tied to a fixed path—could make session creation predictable and avoid unwanted worktrees. Users who prefer isolation would see no behavior change.

### Installation context

GitHub Copilot app desktop; local repository project sessions.

### Additional context

The session-creation API already exposes the required behavior through `workspace_type`, where `worktree` is the default and `branch` uses the existing checkout. This request is for a persistent app-wide default that feeds that existing option.

Start with the session-creation API and its existing workspace_type handling, then trace creation from the UI, chat, issues, pull requests, and scheduled workflows. Done means a persistent app-wide preference defaults to worktree or branch when no explicit choice is supplied, explicit choices still win, and the existing conflict warning or confirmation remains in effect.

索引モデルが issue の本文から書いたものです。