add install command to CLI (#565)

### TL;DR

Added a new `install` command to the MCP Agent CLI for adding MCP servers to client applications.

### What changed?

- Added a new `install` command that provides end-to-end installation of MCP servers to client applications
- Supports multiple client applications: VSCode, Claude Code, Cursor, Claude Desktop, and ChatGPT
- Handles authentication, server configuration, and writing server configuration to client config files
- Automatically detects required secrets and configures them
- Provides options for customizing server name, using existing secrets files, and forcing overwrites

### How to test?

```
# Install to VSCode
mcp-agent install https://api.example.com/servers/my-server/mcp --client vscode

# Install to Claude Code with custom name
mcp-agent install app-id-123 --client claude_code --name my-server

# Install with existing secrets file
mcp-agent install my-server --client cursor --secrets-file secrets.yaml

# Dry run to validate without making changes
mcp-agent install my-server --client claude_desktop --dry-run
```

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

## Release Notes

- **New Features**
    - Added `install` CLI command to register MCP servers with supported client applications (Claude Desktop, VSCode, Cursor, ChatGPT, Claude Code)
    - Supports dry-run preview mode and force overwrite option for existing configurations
- **Tests**
    - Added comprehensive test coverage for install command validation and client-specific configurations

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: jtcorbett

src/mcp_agent/cli/commands/__init__.py

@@ -22,6 +22,7 @@
     configure,
     go,
     check,
+    install,
 )  # noqa: F401
 
 __all__ = [
@@ -41,4 +42,5 @@
     "configure",
     "go",
     "check",
+    "install",
 ]