Address PR review feedback

- Convert CliAuth getters to methods (getInstanceName, getBaseUrl) so
  options can be added in the future
- Remove StoredInstance from cli-node public API, hiding instance details
- Move secretStore to cli-internal for re-use, refactoring from fs-extra
  to node:fs
- Add shared getAuthInstanceService helper in cli-internal for
  constructing secret-store service keys
- Define StoredInstance locally in cli-module-auth instead of importing
  from cli-node
- Update all consumers and tests for the new method-based API

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

packages/cli-module-auth/package.json

@@ -19,6 +19,7 @@
   "license": "Apache-2.0",
   "main": "src/index.ts",
   "types": "src/index.ts",
+  "bin": "bin/backstage-cli-module-auth",
   "files": [
     "dist",
     "bin"
@@ -50,6 +51,5 @@
   },
   "optionalDependencies": {
     "keytar": "^7.9.0"
-  },
-  "bin": "bin/backstage-cli-module-auth"
+  }
 }