FEAT-06: --metadata flag to include file size, line count, and language

[subhan-f]

Add an optional --metadata flag that extends each file entry from a plain string to a structured object:

{
  "src/cli.ts": {
    "content": "...",
    "size": 842,
    "lines": 31,
    "modified": "2026-05-14T10:22:00Z",
    "language": "TypeScript"
  }
}

Useful when feeding output to tools that need structured file information, or when building automated pipelines on top of excod.

Priority: Medium