Detect and preserve existing citation metadata

## Summary

Review how OpenCite should behave when a repository already contains citation metadata.

## Motivation

Many repositories already include a `CITATION.cff` or `.zenodo.json` file. The importer should avoid overwriting valid information with lower-confidence metadata from the GitHub API.

## Tasks

- Document current behavior when citation files already exist
- Identify which fields should always be preserved
- Identify which fields can safely be refreshed from GitHub
- Define precedence rules for imported metadata
- Create follow-up implementation issues if changes are needed

## Acceptance Criteria

- Metadata precedence rules are documented
- Follow-up implementation work is identified

Review the importer’s current handling of existing CITATION.cff and .zenodo.json files, including metadata sourced from the GitHub API. Document which fields are preserved or refreshed, define precedence rules, and identify any follow-up implementation issues needed to meet the acceptance criteria.

Written by the indexing model from the issue text.