docs: add register-gemini-enterprise CLI documentation (#431)

* docs: add register-gemini-enterprise CLI documentation

Add comprehensive documentation for the register-gemini-enterprise command,
including usage examples, parameter descriptions, and troubleshooting guide.

- Create new CLI reference page at docs/cli/register_gemini_enterprise.md
- Add register-gemini-enterprise to CLI index page
- Document all command options and environment variables
- Include quick start examples and common troubleshooting scenarios

* docs: make register-gemini-enterprise documentation more concise

Streamline documentation by:
- Combining redundant sections
- Using table format for optional parameters
- Reducing verbose examples
- Condensing troubleshooting section
- Removing repetitive explanations

Reduced from 264 to 108 lines while keeping all essential information.

* docs: add hyperlink to register-gemini-enterprise documentation

Update template README to link to full documentation for the
register-gemini-enterprise command instead of just mentioning env var.

Author: eliasecchig

agent_starter_pack/base_template/README.md

@@ -72,7 +72,7 @@ make install && make playground
 | `make build-frontend` | Build the frontend for production |
 {%- endif %}
 {%- if cookiecutter.is_adk %}
-| `make register-gemini-enterprise` | Register deployed agent to Gemini Enterprise (see Makefile for parameters) |
+| `make register-gemini-enterprise` | Register deployed agent to Gemini Enterprise ([docs](https://googlecloudplatform.github.io/agent-starter-pack/cli/register_gemini_enterprise.html)) |
 {%- endif %}
 {%- endif %}
 | `make test`          | Run unit and integration tests                                                              |

docs/cli/index.md

@@ -8,5 +8,6 @@ The Agent Starter Pack provides a command-line interface (CLI) to create and set
 - [`setup-cicd`](setup_cicd.md) - Set up CI/CD pipeline for your project
 - [`enhance`](enhance.md) - Add agent-starter-pack capabilities to existing projects without creating a new directory
 - [`list`](list.md) - List available agents and templates
+- [`register-gemini-enterprise`](register_gemini_enterprise.md) - Register a deployed Agent Engine to Gemini Enterprise
 
 For detailed usage instructions, click on the command links above.

docs/cli/register_gemini_enterprise.md

@@ -0,0 +1,107 @@
+# `register-gemini-enterprise`
+
+Register a deployed Agent Engine to Gemini Enterprise, making it available as a tool within the Gemini Enterprise application.
+
+## Usage
+
+```bash
+# Via Makefile (recommended)
+ID="projects/.../engines/xxx" make register-gemini-enterprise
+
+# Direct command
+uvx --from agent-starter-pack agent-starter-pack-register-gemini-enterprise [OPTIONS]
+```
+
+## Quick Start
+
+After deploying your agent, register it with just the Gemini Enterprise app ID. The agent engine ID is automatically read from `deployment_metadata.json`:
+
+```bash
+make deploy  # Creates deployment_metadata.json
+
+ID="projects/123456/locations/global/collections/default_collection/engines/my-engine" \
+  make register-gemini-enterprise
+```
+
+The command automatically:
+- Reads agent engine ID from `deployment_metadata.json`
+- Fetches display name and description from the deployed Agent Engine
+- Creates or updates the registration in Gemini Enterprise
+
+## Prerequisites
+
+- Deployed Agent Engine (creates `deployment_metadata.json`)
+- Gemini Enterprise application configured in Google Cloud
+- Authentication: `gcloud auth application-default login`
+
+## Parameters
+
+### Required
+
+**`--gemini-enterprise-app-id`** (env: `ID`, `GEMINI_ENTERPRISE_APP_ID`)
+
+Gemini Enterprise app resource name.
+
+Format: `projects/{project_number}/locations/{location}/collections/{collection}/engines/{engine_id}`
+
+Note: Use project **number** (numeric), not project ID (string).
+
+Find it: Cloud Console > Discovery Engine > Apps > [Your App] > Details
+
+### Optional
+
+| Parameter | Environment Variable | Default | Description |
+|-----------|---------------------|---------|-------------|
+| `--agent-engine-id` | `AGENT_ENGINE_ID` | From `deployment_metadata.json` | Agent Engine resource name |
+| `--metadata-file` | - | `deployment_metadata.json` | Path to deployment metadata file |
+| `--display-name` | `GEMINI_DISPLAY_NAME` | From Agent Engine or "My Agent" | Display name in Gemini Enterprise |
+| `--description` | `GEMINI_DESCRIPTION` | From Agent Engine or "AI Agent" | Agent description |
+| `--tool-description` | `GEMINI_TOOL_DESCRIPTION` | Same as `--description` | Tool description for Gemini Enterprise |
+| `--project-id` | `GOOGLE_CLOUD_PROJECT` | Extracted from agent-engine-id | GCP project ID for billing |
+| `--authorization-id` | `GEMINI_AUTHORIZATION_ID` | None | OAuth authorization resource name |
+
+## Examples
+
+**Basic registration:**
+```bash
+ID="projects/123456789/locations/global/collections/default_collection/engines/my-engine" \
+  make register-gemini-enterprise
+```
+
+**With custom metadata:**
+```bash
+ID="projects/.../engines/xxx" \
+  GEMINI_DISPLAY_NAME="Support Agent" \
+  GEMINI_DESCRIPTION="Customer support assistant" \
+  make register-gemini-enterprise
+```
+
+**Using environment variables:**
+```bash
+export GEMINI_ENTERPRISE_APP_ID="projects/.../engines/xxx"
+export GEMINI_DISPLAY_NAME="Product Support Agent"
+export GEMINI_DESCRIPTION="AI agent for product support"
+
+agent-starter-pack-register-gemini-enterprise
+```
+
+## Troubleshooting
+
+**"No agent engine ID provided and deployment_metadata.json not found"**
+- Run `make deploy` first to create the metadata file, or provide `--agent-engine-id` explicitly
+
+**"Invalid GEMINI_ENTERPRISE_APP_ID format"**
+- Ensure format: `projects/{project_number}/locations/{location}/collections/{collection}/engines/{engine_id}`
+- Use project **number** (numeric), not project ID
+
+**"Could not access secret with service account"**
+- Grant Cloud Build service account the `secretmanager.secretAccessor` role
+
+**Authentication errors**
+- Run: `gcloud auth application-default login`
+
+## See Also
+
+- [Agent Engine Deployment Guide](../guide/deployment.md)
+- [Gemini Enterprise Integration](https://cloud.google.com/discovery-engine/docs)
+- [CLI Reference](index.md)