[FEATURE] @ file picker should support browsing into nested git repositories

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When running Claude Code from a parent directory that contains nested git repositories (e.g., a monorepo-style setup), the @ file picker only shows top-level entries and does not allow drilling into subdirectories that are their own git repos.

Current Behavior

The @ picker stops at the git boundary of nested repos

Proposed Solution

The @ picker should allow recursive browsing into subdirectories even if they are independent git repositories, so users can reference files across the full workspace.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_