[FEATURE]: support multi-repository change tracking for workspace subdirectories

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Describe the enhancement you want to request

Problem Statement

Currently, the Changes / Source Control panel only tracks Git status when the root workspace folder itself is a Git repository (.git exists at the root).

When working on multi-repository projects (e.g., an organization workspace with org/service-a, org/service-b), opening the parent directory disables git change tracking altogether.

Use Case

In microservices, multi-repo architectures, or full-stack setups, developers often open a parent directory to manage multiple related projects simultaneously. Requiring developers to switch workspace roots repeatedly to check Git status introduces significant context-switching friction.

Proposed Solutions

1. Repository Auto-Discovery (Recommended):
   • Scan subdirectories (up to 1–2 levels deep) for .git folders upon workspace loading.
   • Display detected repositories as grouped trees inside the Changes view (e.g., VS Code multi-root SCM panel style).
2. Fallback / Manual Configuration:
   • If auto-discovery is too heavy, allow defining sub-repository paths in workspace configuration (e.g., .opencode/workspace.json).

Performance Considerations

To avoid performance degradation when scanning deep or large directory trees:

• Limit recursive search depth to a configurable maximum (e.g., depth: 2).
• Honor ignore rules (.gitignore, node_modules, .terragrunt-cache, etc.) during discovery.
• Execute git status lazily or in parallel for detected repos.