feat: add GitHub knowledge source ingestion

[manthan787]

Summary

• implement GitHub knowledge source ingestion using repository tree + blob APIs
• support both owner/repo and repository URL inputs
• parse markdown/yaml/text documents into normalized knowledge documents with chunking and metadata inference
• wire github source type in the knowledge source dispatcher
• add focused unit tests for GitHub loading and invalid repo handling

Validation

• npm run test -- src/knowledge/sources/__tests__/github.test.ts
• npm run typecheck