feat: add gemini-cli to x1 system and overlays

- Introduced `gemini-cli` as a CLI tool in the x1 configuration.
- Added a new overlay for `gemini-cli` package with npm dependencies.
- Removed `goose-cli` and `aider-chat` from the existing application list.

overlays/unstable/gemini-cli/update.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p nodePackages.npm nix-update
+
+set -euo pipefail
+
+version=$(npm view @google/gemini-cli version)
+
+# Generate updated lock file
+cd "$(dirname "${BASH_SOURCE[0]}")"
+npm i --package-lock-only @google/gemini-cli@"$version"
+rm -f package.json
+
+# Update version and hashes
+cd -
+nix-update gemini-cli --version "$version"
+