fix: store base content for deleted PR files

[CaptainTimon]

Summary

• Store base-side content for deleted PR files instead of treating removed files as already complete.
• Avoid head-side blob lookups for removed files, so deleted files persist base_content while head_content remains null.
• Add focused regression coverage for removed-file content batching without changing package scripts, dependencies, or build behavior.

Related Issues

Fixes #20

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• git diff --check
• node --test -r ts-node/register src/webhook/github-fetcher.service.spec.ts
• npm run format:check
• npm run lint
• npm run build

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable)
• Documentation updated (if applicable)
• No unnecessary dependencies added