fix: migrate extension commands on integration switch (#2404)

* fix: migrate extension commands on integration switch

When switching integrations (e.g. kimi → opencode), extension commands
were not re-registered for the new agent, leaving the new agent without
extension support and orphaning files in the old agent's directory.

Changes:
- Add ExtensionManager.unregister_agent_artifacts() to clean up old
  agent extension files and registry entries during switch
- Add ExtensionManager.register_enabled_extensions_for_agent() to
  re-register all enabled extensions for the new agent
- Wire both into integration_switch() after uninstall/install phases
- Handle skills mode (Copilot --skills) correctly
- Add tests for kimi→opencode→claude migration, Copilot skills mode,
  and disabled extension handling

Fixes extension commands not appearing after integration switch.

* Update src/specify_cli/extensions.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/specify_cli/__init__.py

@@ -2376,6 +2376,19 @@ def integration_switch(
             )
             raise typer.Exit(1)
 
+        # Unregister extension commands for the old agent so they don't
+        # remain as orphans in the old agent's directory.
+        try:
+            from .extensions import ExtensionManager
+
+            ext_mgr = ExtensionManager(project_root)
+            ext_mgr.unregister_agent_artifacts(installed_key)
+        except Exception as ext_err:
+            console.print(
+                f"[yellow]Warning:[/yellow] Could not clean up extension artifacts "
+                f"(commands, skills, registry entries) for '{installed_key}': {ext_err}"
+            )
+
         # Clear metadata so a failed Phase 2 doesn't leave stale references
         _remove_integration_json(project_root)
         opts = load_init_options(project_root)
@@ -2415,6 +2428,19 @@ def integration_switch(
         _write_integration_json(project_root, target_integration.key)
         _update_init_options_for_integration(project_root, target_integration, script_type=selected_script)
 
+        # Re-register extension commands for the new agent so that
+        # previously-installed extensions are available in the new integration.
+        try:
+            from .extensions import ExtensionManager
+
+            ext_mgr = ExtensionManager(project_root)
+            ext_mgr.register_enabled_extensions_for_agent(target)
+        except Exception as ext_err:
+            console.print(
+                f"[yellow]Warning:[/yellow] Could not register extension commands, skills, "
+                f"or related artifacts for '{target}': {ext_err}"
+            )
+
     except Exception as e:
         # Attempt rollback of any files written by setup
         try: