feat: add reference agent for searching external repositories

[thdxr]

Summary

• Adds ability to configure external git repositories or local paths in opencode.json under a new references field
• Creates a reference subagent that can search across referenced projects using glob, grep, and read tools
• Git references are automatically cloned (shallow) to a cache directory and kept fresh (stale threshold: 1 hour)
• Supports branch specification via url#branch syntax

Example Configuration

{
  "references": [
    "git@github.com:Effect-TS/effect.git",
    "https://github.com/some/repo#main"
  ]
}

This enables querying external codebases without needing to clone them locally or add them as workspace dependencies.