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>

tests/test_model_pricing_usage_cycle.py

@@ -0,0 +1,60 @@
+"""Task 4a — model_pricing <-> usage_tracker circular dependency is gone.
+
+Before this fix, ``model_pricing.turn_cost_usd`` deferred-imported
+``usage_tracker`` for its flat fallback rates, while ``usage_tracker.set_budget``
+deferred-imported ``model_pricing`` for currency conversion — a real
+architectural cycle, only avoided at runtime by pushing both imports inside
+function bodies. Now ``model_pricing`` is a leaf module (it owns its own
+fallback rates) and ``usage_tracker`` imports it at module top level.
+"""
+from __future__ import annotations
+
+import copy
+import sys
+
+from cowork_local.config import AppConfig, DEFAULT_CONFIG
+from cowork_local.core import model_pricing, usage_tracker
+
+
+def test_model_pricing_does_not_depend_on_usage_tracker_module_level() -> None:
+    assert "usage_tracker" not in model_pricing.__dict__
+    assert "usage_tracker" not in getattr(model_pricing, "__all__", [])
+
+
+def test_usage_tracker_imports_model_pricing_at_top_level() -> None:
+    assert usage_tracker.mp is model_pricing
+
+
+def test_turn_cost_usd_fallback_matches_pre_refactor_default_rates(tmp_path) -> None:
+    config = AppConfig(data=copy.deepcopy(DEFAULT_CONFIG), path=tmp_path / "config.json")
+    # No matching row in the price table and no override in config.data["usage"]
+    # -> falls back to the flat rates that used to live in
+    # usage_tracker.DEFAULT_PRICING (0.5 in / 1.5 out USD per 1M tokens).
+    cost = model_pricing.turn_cost_usd("some-unknown-model", 1_000_000, 1_000_000, config)
+    assert cost == 0.5 + 1.5
+
+
+def test_turn_cost_usd_honours_usage_override_like_before(tmp_path) -> None:
+    config = AppConfig(data=copy.deepcopy(DEFAULT_CONFIG), path=tmp_path / "config.json")
+    config.data.setdefault("usage", {})["price_per_mtok_in_usd"] = 2.0
+    config.data["usage"]["price_per_mtok_out_usd"] = 4.0
+    cost = model_pricing.turn_cost_usd("some-unknown-model", 1_000_000, 1_000_000, config)
+    assert cost == 2.0 + 4.0
+
+
+def test_set_budget_still_converts_via_model_pricing(tmp_path) -> None:
+    config = AppConfig(data=copy.deepcopy(DEFAULT_CONFIG), path=tmp_path / "config.json")
+    usage_tracker.set_budget(config, 100.0, currency="USD")
+    status = usage_tracker.budget_status(config)
+    assert status is not None
+    assert status["amount_usd"] == 100.0
+
+
+def test_no_import_time_cycle_when_loaded_fresh() -> None:
+    for name in ("cowork_local.core.model_pricing", "cowork_local.core.usage_tracker"):
+        sys.modules.pop(name, None)
+    import importlib
+
+    mp = importlib.import_module("cowork_local.core.model_pricing")
+    ut = importlib.import_module("cowork_local.core.usage_tracker")
+    assert ut.mp is mp