[Feature] Support LLM-specific output formatting (e.g., XML tags for Claude)

[ImmaZoni]

Description

Currently, files are formatted using standard Markdown headers (## src/main.go). However, foundational models—especially Anthropic's Claude—perform significantly better at code-retrieval tasks when files are wrapped in specific XML tags.

Why it matters

Using the formatting native to the target LLM reduces hallucinations and improves the model's ability to accurately cite the exact file it is referencing.

Proposed Solution

Offer an --xml or --format claude flag that outputs files like this:

<file name="src/main.go">
// code here
</file>```

### Tasks

- [ ] Add `--format` flag (options: `markdown`, `xml`, `json`, etc.).
- [ ] Update `processFileContent` to conditionally format the wrapper based on the selected format.