Improve run_command error diagnostics with live output capture

Switch run_command to a single-run execution model that streams stdout/stderr
live while capturing both streams in memory using selectors. This guarantees
that command errors (e.g. make install, pip, nix) always show full diagnostics
without re-running commands or risking deadlocks.

Add unit tests for preview mode, success execution, failure handling, and
allow_failure behavior.

Context:
https://chatgpt.com/share/replace-with-this-conversation-link

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

@@ -0,0 +1,47 @@
+import unittest
+from unittest.mock import patch
+
+import pkgmgr.core.command.run as run_mod
+
+
+class TestRunCommand(unittest.TestCase):
+    def test_preview_returns_success_without_running(self) -> None:
+        with patch.object(run_mod.subprocess, "Popen") as popen_mock:
+            result = run_mod.run_command("echo hi", cwd="/tmp", preview=True)
+            self.assertEqual(result.returncode, 0)
+            popen_mock.assert_not_called()
+
+    def test_success_streams_and_returns_completed_process(self) -> None:
+        cmd = ["python3", "-c", "print('out'); import sys; print('err', file=sys.stderr)"]
+
+        with patch.object(run_mod.sys, "exit") as exit_mock:
+            result = run_mod.run_command(cmd, allow_failure=False)
+
+        self.assertEqual(result.returncode, 0)
+        self.assertIn("out", result.stdout)
+        self.assertIn("err", result.stderr)
+        exit_mock.assert_not_called()
+
+    def test_failure_exits_when_not_allowed(self) -> None:
+        cmd = ["python3", "-c", "import sys; print('oops', file=sys.stderr); sys.exit(2)"]
+
+        with patch.object(run_mod.sys, "exit", side_effect=SystemExit(2)) as exit_mock:
+            with self.assertRaises(SystemExit) as ctx:
+                run_mod.run_command(cmd, allow_failure=False)
+
+        self.assertEqual(ctx.exception.code, 2)
+        exit_mock.assert_called_once_with(2)
+
+    def test_failure_does_not_exit_when_allowed(self) -> None:
+        cmd = ["python3", "-c", "import sys; print('oops', file=sys.stderr); sys.exit(3)"]
+
+        with patch.object(run_mod.sys, "exit") as exit_mock:
+            result = run_mod.run_command(cmd, allow_failure=True)
+
+        self.assertEqual(result.returncode, 3)
+        self.assertIn("oops", result.stderr)
+        exit_mock.assert_not_called()
+
+
+if __name__ == "__main__":
+    unittest.main()