Feature/delta team/epic r04 (#7)

## Summary

epic r04 - begin refactor

## Change Type

- [x] Cowork feature
- [ ] Bug fix
- [ ] Core AI contribution
- [ ] Test / hardening
- [ ] Performance
- [ ] Documentation

## Related Work

Cowork Task:

Core Repo: http://34.143.229.138/gitea-admin/fsg-ai-core-assets

Core AI Issue:

Core Task:

Related PR:

## Scope

What is intentionally included?

What is intentionally NOT included?

## Validation

- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual verification
- [ ] Regression check

Commands / evidence:

## Security Impact

Permission / credential / network / customer data impact:

## Compatibility

- [ ] No breaking change
- [ ] Breaking change documented

## Reviewer Notes

Anything Cowork reviewers should pay attention to.

---------

Co-authored-by: Anh Tran Nguyen Minh <anhtnm1@fpt.com>
Co-authored-by: Huong Le Thi Thien <huongltt35@fpt.com>
Co-authored-by: Nam Pham Dinh Thanh <nampdt@fpt.com>
Co-authored-by: Vu Dam Tuan <vudt15@fpt.com>
Co-authored-by: Hiep Ha Van <hiephv3@fpt.com>
Co-authored-by: Lam Hoang Van <lamhv7@fpt.com>
Reviewed-on: #7
Co-authored-by: Duy Le Huu <duylh19@fpt.com>

core/routing/selector.py

@@ -27,6 +27,7 @@ class RankedCandidate:
 
     @property
     def key(self) -> str:
+        """Khoá định danh của ứng viên (provider + model)."""
         return self.assessment.key
 
 
@@ -40,6 +41,7 @@ class Ranking:
 
     @property
     def best(self) -> Optional[RankedCandidate]:
+        """Ứng viên đứng đầu; ``None`` nếu không có ứng viên nào."""
         return self.ranked[0] if self.ranked else None
 
     def score_of(self, key: str) -> float:
@@ -66,6 +68,7 @@ class Ranking:
 
 
 def _has_capabilities(assessment: ModelAssessment, required: Set[str]) -> bool:
+    """Model này có đủ mọi năng lực mà lượt chạy đòi hỏi không."""
     return required.issubset(assessment.metadata.capabilities)
 
 
@@ -99,6 +102,10 @@ def rank_models(
         scored.append(RankedCandidate(assessment=a, score=score))
 
     def _sort_key(c: RankedCandidate):
+        """Khoá sắp xếp ứng viên: điểm cao trước, cùng điểm thì rẻ hơn trước.
+
+        Model chưa biết giá bị xếp cuối (coi như vô cùng đắt) chứ không phải miễn phí.
+        """
         cost = c.assessment.metadata.avg_cost_per_1k
         cost = cost if cost is not None else float("inf")
         # score desc, then cheaper, then model id for determinism.