Fetch base content for deleted PR files

[MkDev11]

Summary

• Fetch base-side blob contents for removed PR files instead of filtering them out of PR file content fetching.
• Skip head-side blob lookups for removed files, so deleted files persist base_content while head_content remains null.
• Keep the patch focused on the accepted github-fetcher.service.ts change with no contributor test additions.

Related Issues

Closes #20

Type of Change

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

Testing

• npm run format:check
• npm run lint
• npm run build
• git diff --check

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

[anderdc]

This repo doesn't accept tests in contributor PRs. Drop packages/das/src/webhook/github-fetcher.service.spec.ts. The github-fetcher.service.ts change itself is fine.

[MkDev11]

This repo doesn't accept tests in contributor PRs. Drop packages/das/src/webhook/github-fetcher.service.spec.ts. The github-fetcher.service.ts change itself is fine.

done