Support multi-project workspace with @ mentions (like Cursor)

Problem

Currently Claude Code only supports operating within a single project directory. Unlike Cursor (which uses VS Code's multi-root workspace feature), there's no way to work with multiple projects simultaneously in a single conversation.

Desired Behavior

Support multi-project workspace similar to Cursor's approach:

1. Allow opening multiple project directories in one session
2. Use @ syntax to reference code from different projects
3. Claude indexes all open projects, enabling cross-project code understanding

Use Case

For monorepo-style setups or when two projects call each other, developers need Claude to understand both codebases. Currently requires:

• Opening two separate Claude Code instances
• Manually referencing files with absolute paths (inefficient)

Why This Matters

Cursor's @ workspace feature is extremely useful for:

• Cross-project refactoring
• Understanding API contracts between services
• Building shared SDKs that span multiple repos)