core(command): implement explicit command=None bypass and add unit tests

This update introduces Variant B behavior in the command resolver:

- If a repository explicitly defines the key \"command\" (even if its value is None),
  resolve_command_for_repo() treats it as authoritative and returns immediately.
  This allows library-only repositories to declare:
      command: null
  which disables CLI resolution entirely.

- As a result, Python package repositories without installed CLI entry points
  no longer trigger SystemExit during update/install flows, as long as they set
  command: null in their repo configuration.

The resolution logic is now bypassed for such repositories, skipping:
  - Python package detection (src/*/__main__.py)
  - PATH/Nix/venv binary lookup
  - main.sh/main.py fallback evaluation

A new unit test suite has been added under
  tests/unit/pkgmgr/core/command/test_resolve.py
covering:

 1) Python package without installed command → SystemExit
 2) Python package with installed command → returned correctly
 3) Script repository fallback to main.py
 4) Explicit command overrides all logic

This commit stabilizes update/install flows and ensures library-only
repositories behave as intended when no CLI command is provided.

https://chatgpt.com/share/69394a53-bc78-800f-995d-21099a68dd60

tests/unit/pkgmgr/core/command/test_resolve.py

@@ -0,0 +1,93 @@
+import os
+import shutil
+import tempfile
+import unittest
+from unittest.mock import patch
+
+from pkgmgr.core.command.resolve import resolve_command_for_repo
+
+
+class TestResolveCommandForRepo(unittest.TestCase):
+
+    # ----------------------------------------------------------------------
+    # Helper: Create a fake src/<pkg>/__main__.py for Python package detection
+    # ----------------------------------------------------------------------
+    def _create_python_package(self, repo_dir, package_name="mypkg"):
+        src = os.path.join(repo_dir, "src", package_name)
+        os.makedirs(src, exist_ok=True)
+        main_file = os.path.join(src, "__main__.py")
+        with open(main_file, "w", encoding="utf-8") as f:
+            f.write("# fake python package entry\n")
+        return main_file
+
+    # ----------------------------------------------------------------------
+    # 1) Python package but no installed command → must fail with SystemExit
+    # ----------------------------------------------------------------------
+    def test_python_package_without_installed_command_raises(self):
+        with tempfile.TemporaryDirectory() as repo_dir:
+
+            # Fake Python package src/.../__main__.py
+            self._create_python_package(repo_dir)
+
+            repo = {}
+            repo_identifier = "analysis-ready-code"
+
+            with patch("shutil.which", return_value=None):
+                with self.assertRaises(SystemExit) as ctx:
+                    resolve_command_for_repo(repo, repo_identifier, repo_dir)
+
+            self.assertIn("Python package", str(ctx.exception))
+
+    # ----------------------------------------------------------------------
+    # 2) Python package with installed command via PATH → returns command
+    # ----------------------------------------------------------------------
+    def test_python_package_with_installed_command(self):
+        with tempfile.TemporaryDirectory() as repo_dir:
+
+            # Fake python package
+            self._create_python_package(repo_dir)
+
+            repo = {}
+            repo_identifier = "analysis-ready-code"
+
+            fake_binary = os.path.join(repo_dir, "fakebin", "analysis-ready-code")
+            os.makedirs(os.path.dirname(fake_binary), exist_ok=True)
+            with open(fake_binary, "w") as f:
+                f.write("#!/bin/sh\necho test\n")
+            os.chmod(fake_binary, 0o755)
+
+            with patch("shutil.which", return_value=fake_binary):
+                result = resolve_command_for_repo(repo, repo_identifier, repo_dir)
+                self.assertEqual(result, fake_binary)
+
+    # ----------------------------------------------------------------------
+    # 3) Script repo: return main.py if present
+    # ----------------------------------------------------------------------
+    def test_script_repo_fallback_main_py(self):
+        with tempfile.TemporaryDirectory() as repo_dir:
+
+            fake_main = os.path.join(repo_dir, "main.py")
+            with open(fake_main, "w", encoding="utf-8") as f:
+                f.write("# script\n")
+
+            repo = {}
+            repo_identifier = "myscript"
+
+            with patch("shutil.which", return_value=None):
+                result = resolve_command_for_repo(repo, repo_identifier, repo_dir)
+                self.assertEqual(result, fake_main)
+
+    # ----------------------------------------------------------------------
+    # 4) Explicit command has highest priority
+    # ----------------------------------------------------------------------
+    def test_explicit_command(self):
+        with tempfile.TemporaryDirectory() as repo_dir:
+            repo = {"command": "/custom/runner.sh"}
+            repo_identifier = "x"
+
+            result = resolve_command_for_repo(repo, repo_identifier, repo_dir)
+            self.assertEqual(result, "/custom/runner.sh")
+
+
+if __name__ == "__main__":
+    unittest.main()