Desktop app: support scoping to subfolder in monorepo

Problem

In the Mac desktop app, there's no way to scope Claude Code's context to a specific subfolder within a monorepo. The project context is always tied to the git root, which means the entire monorepo context loads every time.

Use case

Working on individual packages/apps in a monorepo (e.g., apps/figma-plugins/my-plugin/ inside a larger monorepo). Users want to:

• Only load CLAUDE.md and context relevant to that subfolder
• Not have the full monorepo context in every request
• Quickly switch between different packages/apps in the monorepo

Current workaround

The only reliable option is using the CLI and cd-ing into the subfolder before running claude. This isn't available in the desktop app.

Suggestion

Allow the desktop app to set a "working directory" or "project scope" to a subfolder within a monorepo, similar to how VS Code workspaces or other tools handle monorepo subfolder focus. Other AI coding tools handle this better today.