feat: Optionally install extensions (#329)

**Summary**

This PR introduces the optional installation of extensions

**Changes**
- New `extensions` input: A newline separated list of extension
repository URLs.
- Conditional installation: The install workflow checks the extension
flag and installs extensions that are listed

Author: CallumHYoung

README.md

@@ -179,6 +179,8 @@ go to the [Gemini Assistant workflow documentation](./examples/workflows/gemini-
 -   <a name="use_vertex_ai"></a><a href="#user-content-use_vertex_ai"><code>use_vertex_ai</code></a>: _(Optional, default: `false`)_ Whether to use Vertex AI for Gemini model access instead of the default Gemini API key.
     For more information, see the [Gemini CLI documentation](https://github.com/google-gemini/gemini-cli/blob/main/docs/cli/authentication.md).
 
+-   <a name="extensions"></a><a href="#user-content-extensions"><code>extensions</code></a>: _(Optional)_ A list of Gemini CLI extensions to install.
+
 
 <!-- END_AUTOGEN_INPUTS -->
 

action.yml

@@ -68,6 +68,9 @@ inputs:
       For more information, see the [Gemini CLI documentation](https://github.com/google-gemini/gemini-cli/blob/main/docs/cli/authentication.md).
     required: false
     default: 'false'
+  extensions:
+    description: 'A list of Gemini CLI extensions to install.'
+    required: false
 
 outputs:
   summary:
@@ -195,6 +198,7 @@ runs:
       id: 'install'
       env:
         GEMINI_CLI_VERSION: '${{ inputs.gemini_cli_version }}'
+        EXTENSIONS: '${{ inputs.extensions }}'
       shell: 'bash'
       run: |-
         set -euo pipefail
@@ -220,6 +224,16 @@ runs:
           echo "Error: Gemini CLI not found in PATH"
           exit 1
         fi
+        if [[ -n "${EXTENSIONS}" ]]; then
+          echo "Installing Gemini CLI extensions:"
+          echo "${EXTENSIONS}" | jq -r '.[]' | while IFS= read -r extension; do
+            extension=$(echo "${extension}" | xargs)
+            if [[ -n "${extension}" ]]; then
+              echo "Installing ${extension}..."
+              echo "Y" | gemini extensions install --source "${extension}"
+            fi
+          done
+        fi
 
     - name: 'Run Gemini CLI'
       id: 'gemini_run'