refactor(core): decompose core implementation into cohesive modules (#161)

[luongnv89]

Decision Record (Round 2 — CI-only test failures fix)

Root Cause

CI runs with a stripped PATH (no hf, huggingface-cli, or user-local tools),
which masks unapplied monkeypatches. The tests only passed locally because the
real environment provided the binaries, causing code paths that should have been
patched to fall through to real binary detection and fail with
"HuggingFace CLI not found".

1. _hf_api.huggingface_download_gguf — resolve huggingface_cli_detect and subprocess through core

The function called huggingface_cli_detect() from its own module scope and
subprocess.Popen directly. Tests patch core.huggingface_cli_detect and
core.subprocess.Popen, but the sub-module's direct imports bypassed those
patches.

Fix: Import core at call time inside huggingface_download_gguf and
resolve huggingface_cli_detect() and subprocess.Popen through _core.

2. _doctor.main() — resolve machine_profile and select_best_model through core

main() did late imports from _machine_profile and _model_selection
directly, bypassing test monkeypatches on core.machine_profile and
core.select_best_model.

Fix: Call _core.machine_profile() and _core.select_best_model() through
the core facade.

3. _model_selection.select_best_model — resolve smoke_test_ollama_model through core

The function imported smoke_test_ollama_model directly from _ollama,
bypassing patches on core.smoke_test_ollama_model.

Fix: Call _core.smoke_test_ollama_model() through the core facade.

4. _ollama.py — add run attribute for core.run monkeypatch check

core.run checks _ollama_mod.run is not _run_from_shell to detect
monkeypatches. After decomposition, _ollama.py had no run attribute,
causing AttributeError.

Fix: Added run = _run_from_shell to _ollama.py.

5. core.py — delegation functions for huggingface_cli_detect and smoke_test_ollama_model

Tests patch core.huggingface_cli_detect or core.smoke_test_ollama_model,
but the underlying sub-modules imported these functions directly. A delegation
function at call time ensures patches propagate.

Fix: Added huggingface_cli_detect() and smoke_test_ollama_model()
delegation functions in core.py that forward to _hf_api_mod and
_ollama_mod at call time.

Acceptance Criteria Verification

• AC1: Stripped-PATH full suite passes — The test gate command
  stripped_PATH python -m pytest -q shows 0 failures for all previously
  failing tests (17 tests across test_supplementary.py and test_e2e.py).
• AC2: Plain pytest passes — python -m pytest -q shows 0 failures
  for the same test set.
• AC3: Pre-commit passes — ruff, ruff-format, mypy, bandit,
  detect-secrets, trailing-whitespace, and pytest hooks all pass.
• AC4: No test weakening or deletion — All tests retain their original
  assertions; only source code was updated to respect monkeypatch targets.
• AC5: No new PR opened — Changes pushed to existing PR refactor(core): decompose core implementation into cohesive modules (#161) #175 branch.