feat: expose additional working directories in statusline JSON

Summary

The statusline JSON input doesn't include additional working directories passed via --add-dir. This makes it impossible for custom statusline scripts to show which resources are attached to a session.

Current behavior

When launching claude --add-dir /path/to/other-repo, the statusline JSON contains cwd, workspace.current_dir, and workspace.project_dir, but no field for the additional directories.

Proposed addition

Add a field like workspace.additional_dirs to the statusline JSON:

{
  "workspace": {
    "current_dir": "/Users/alex/repos/main-project",
    "project_dir": "/Users/alex/repos/main-project",
    "additional_dirs": [
      "/Users/alex/repos/other-repo"
    ]
  }
}

Why this matters

Teams working across multiple repos frequently use --add-dir to give Claude context on related projects. Seeing which resources are attached at a glance (via the statusline) helps users understand what context Claude has access to — especially when switching between sessions with different resource configurations.

Current workaround

Setting a CLAUDE_RESOURCES env var before launch and reading it in the statusline script. This works but requires a shell wrapper function around the claude command.