fix: support classic Mac line endings

[vycdev2]

Summary

• Handle classic Mac carriage-return line endings consistently when scanning embedded todos and parsing Ackmate output.
• Reuse one line-splitting helper across the embedded JavaScript provider, ag context loading, pre-scan, and external-search parser.
• Add regression tests for embedded matches and Ackmate records separated by \\r.
• Document the user-visible compatibility fix in the unreleased changelog.

Verification

• Focused regression tests were red before the implementation and green after it.
• npm install — passed; npm reported existing deprecation and vulnerability warnings.
• npx mocha -r ts-node/register test/embedded-regex.spec.ts --grep "classic Mac" --exit — passed.
• npx mocha -r ts-node/register test/ackmate.spec.ts --grep "classic Mac" --exit — passed.
• npm test — passed (136 tests).
• npm run compile — passed.
• npm run format:check — passed.
• git diff --check — passed.

Risk

• Low: LF and CRLF behavior is unchanged; only classic Mac \\r separators are recognized in the affected line-oriented paths.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.