feat(10-01): add shared retry HTTP client for recon sources

- Client.Do retries 429/403/5xx honoring Retry-After
- 401 returns ErrUnauthorized immediately (no retry)
- Context cancellation honored during retry sleeps
- Default UA keyhunter-recon/1.0, 30s timeout, 2 retries

pkg/recon/sources/doc.go

@@ -0,0 +1,4 @@
+// Package sources hosts per-OSINT-source ReconSource implementations for Phase 10
+// code hosting (GitHub, GitLab, Bitbucket, Gist, Codeberg, HuggingFace, Kaggle,
+// Replit, CodeSandbox, sandboxes). Each source implements pkg/recon.ReconSource.
+package sources