fix(extensions): check SKILL collision for auto-corrected aliases

After auto-correcting an alias (e.g. 'speckit.test-ext.hello' →
'test-ext.hello'), the corrected name was written back to aliases[i]
and rename_map but never checked against seen_skill_names nor added
to it. This allowed a corrected alias to silently produce the same
SKILL output name as an existing primary or alias.

Now the corrected name is collision-checked before the warning is
emitted and registered in seen_skill_names immediately after.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: iamaeroplane

src/specify_cli/extensions.py

@@ -265,13 +265,23 @@ def _validate(self):
                 else:
                     corrected = self._try_correct_alias_name(alias, ext["id"])
                     if corrected:
+                        corrected_skill = self._skill_output_name(corrected)
+                        if corrected_skill in seen_skill_names:
+                            raise ValidationError(
+                                f"Alias '{alias}' (corrected to '{corrected}') on command "
+                                f"'{cmd['name']}' would produce SKILL output name "
+                                f"'{corrected_skill}' which is already claimed by "
+                                f"{seen_skill_names[corrected_skill]}. "
+                                f"Choose a distinct alias name."
+                            )
                         self.warnings.append(
                             f"Alias '{alias}' does not follow the required pattern "
                             f"'{{extension}}.{{command}}'. Registering as '{corrected}'. "
                             f"The extension author should update the manifest to use this name."
                         )
                         rename_map[alias] = corrected
                         aliases[i] = corrected
+                        seen_skill_names[corrected_skill] = f"alias '{corrected}' on command '{cmd['name']}'"
                     else:
                         raise ValidationError(
                             f"Invalid alias '{alias}': "

tests/test_extensions.py

@@ -397,6 +397,22 @@ def test_cross_command_skill_name_collision_rejected(self, temp_dir, valid_manif
         with pytest.raises(ValidationError, match="would produce SKILL output name"):
             ExtensionManifest(manifest_path)
 
+    def test_corrected_alias_skill_name_collision_rejected(self, temp_dir, valid_manifest_data):
+        """Auto-corrected alias that collides with an existing primary's SKILL name is rejected."""
+        import yaml
+
+        # Primary 'speckit.test-ext.hello' → skill 'speckit-test-ext-hello'.
+        # Alias 'speckit.test-ext.hello' gets auto-corrected to 'test-ext.hello'
+        # which also maps to 'speckit-test-ext-hello' — collision.
+        valid_manifest_data["provides"]["commands"][0]["aliases"] = ["speckit.test-ext.hello"]
+
+        manifest_path = temp_dir / "extension.yml"
+        with open(manifest_path, "w") as f:
+            yaml.dump(valid_manifest_data, f)
+
+        with pytest.raises(ValidationError, match="would produce SKILL output name"):
+            ExtensionManifest(manifest_path)
+
     def test_hook_alias_ref_canonicalized_to_speckit_form(self, temp_dir, valid_manifest_data):
         """Hook command refs in alias form are lifted to canonical speckit.ext.cmd form."""
         import yaml