cli-node: add CliAuth class for shared CLI authentication

Introduces a class-based authentication management API in @backstage/cli-node
that reads the on-disk instance store, transparently refreshes expired tokens,
and provides a convenient surface for other CLI modules to consume.

The split keeps filesystem-based instance selection and writes owned by
cli-module-auth, while reading and consuming the current instance is
available through CliAuth in cli-node.

Migrates cli-module-actions to use the new API and deprecates the ad-hoc
function exports from cli-module-auth.

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

packages/cli-node/package.json

@@ -52,6 +52,9 @@
     "@backstage/test-utils": "workspace:^",
     "@types/yarnpkg__lockfile": "^1.1.4"
   },
+  "optionalDependencies": {
+    "keytar": "^7.9.0"
+  },
   "peerDependencies": {
     "@swc/core": "^1.15.6"
   },