fix(core/ink): prevent self-referential symlinks + add unit tests

This commit adds a safety guard to create_ink() to prevent creation of
self-referential symlinks when the resolved command already lives at the
intended link target (e.g. ~/.local/bin/package-manager). Such a situation
previously resulted in broken shells with the error:

    "zsh: too many levels of symbolic links"

Key changes:
  - create_ink():
      • Introduce early-abort guard when command == link_path
      • Improve function signature and formatting
      • Enhance alias creation messaging

  - Added comprehensive unit tests under:
        tests/unit/pkgmgr/core/command/test_ink.py
    Tests cover:
      • Self-referential command path → skip symlink creation
      • Standard symlink + alias creation behaviour

This prevents pkgmgr from overwriting user-managed binaries inside ~/.local/bin
and ensures predictable, safe behaviour across all installer layers.

https://chatgpt.com/share/6938a43b-0eb8-800f-9545-6cb555ab406d

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

@@ -0,0 +1,108 @@
+import os
+import tempfile
+import unittest
+from unittest.mock import patch
+
+from pkgmgr.core.command.ink import create_ink
+
+
+class TestCreateInk(unittest.TestCase):
+    @patch("pkgmgr.core.command.ink.get_repo_dir")
+    @patch("pkgmgr.core.command.ink.get_repo_identifier")
+    def test_self_referential_command_skips_symlink(
+        self,
+        mock_get_repo_identifier,
+        mock_get_repo_dir,
+    ):
+        """
+        If the resolved command path is identical to the final link target,
+        create_ink() must NOT replace it with a self-referential symlink.
+
+        This simulates the situation where the command already lives at
+        ~/.local/bin/<identifier> and we would otherwise create a symlink
+        pointing to itself.
+        """
+        mock_get_repo_identifier.return_value = "package-manager"
+        mock_get_repo_dir.return_value = "/fake/repo"
+
+        with tempfile.TemporaryDirectory() as bin_dir:
+            # Simulate an existing real binary at the final link location.
+            command_path = os.path.join(bin_dir, "package-manager")
+            with open(command_path, "w", encoding="utf-8") as f:
+                f.write("#!/bin/sh\necho package-manager\n")
+
+            # Sanity check: not a symlink yet.
+            self.assertTrue(os.path.exists(command_path))
+            self.assertFalse(os.path.islink(command_path))
+
+            repo = {"command": command_path}
+
+            # This must NOT turn the file into a self-referential symlink.
+            create_ink(
+                repo=repo,
+                repositories_base_dir="/fake/base",
+                bin_dir=bin_dir,
+                all_repos=[],
+                quiet=True,
+                preview=False,
+            )
+
+            # After create_ink(), the file must still exist and must not be a symlink.
+            self.assertTrue(os.path.exists(command_path))
+            self.assertFalse(
+                os.path.islink(command_path),
+                "create_ink() must not create a self-referential symlink "
+                "when command == link_path",
+            )
+
+    @patch("pkgmgr.core.command.ink.get_repo_dir")
+    @patch("pkgmgr.core.command.ink.get_repo_identifier")
+    def test_create_symlink_for_normal_command(
+        self,
+        mock_get_repo_identifier,
+        mock_get_repo_dir,
+    ):
+        """
+        In the normal case (command path != link target), create_ink()
+        must create a symlink in bin_dir pointing to the given command,
+        and optionally an alias symlink when repo['alias'] is set.
+        """
+        mock_get_repo_identifier.return_value = "mytool"
+
+        with tempfile.TemporaryDirectory() as repo_dir, tempfile.TemporaryDirectory() as bin_dir:
+            mock_get_repo_dir.return_value = repo_dir
+
+            # Create a fake executable inside the repository.
+            command_path = os.path.join(repo_dir, "main.sh")
+            with open(command_path, "w", encoding="utf-8") as f:
+                f.write("#!/bin/sh\necho mytool\n")
+            os.chmod(command_path, 0o755)
+
+            repo = {
+                "command": command_path,
+                "alias": "mt",
+            }
+
+            create_ink(
+                repo=repo,
+                repositories_base_dir="/fake/base",
+                bin_dir=bin_dir,
+                all_repos=[],
+                quiet=True,
+                preview=False,
+            )
+
+            link_path = os.path.join(bin_dir, "mytool")
+            alias_path = os.path.join(bin_dir, "mt")
+
+            # Main link must exist and point to the command.
+            self.assertTrue(os.path.islink(link_path))
+            self.assertEqual(os.readlink(link_path), command_path)
+
+            # Alias must exist and point to the main link.
+            self.assertTrue(os.path.islink(alias_path))
+            self.assertEqual(os.readlink(alias_path), link_path)
+
+
+if __name__ == "__main__":
+    unittest.main()