Add a gel configure-agent tool

We really want whoever uses Gel with LLMs to use our prompts and the MCP server because they make the experience _a lot_ less frustrating. The setup process for different LLM code tools is very confusing, so Yury has proposed to add a dedicated CLI command to do just that.

## Stuff to be configured

1. The MCP server: [repo](https://github.com/geldata/gel-mcp)

```json
{
"mcpServers": {
"gel": {
"command": "uvx",
"args": [
"--refresh",
"--directory",
".",
"--from",
"git+https://github.com/geldata/gel-mcp.git@refine-rules",
"gel-mcp"
]
}
}
}
```

Anatomy of the command:

- `refresh` to avoid `uv` using the cached old version of the tools.
- `--directory .` to execute from current project's directory (lets Gel connect and run `execute_query` tool).
- `--from ` because we don't actually publish the Python package yet.

2. The Rules prompts (to instruct LLMs to use Gel correctly): [general](https://github.com/geldata/gel-mcp/blob/main/src/gel_mcp/static/src/gel-rules-auto.md), [python](https://github.com/geldata/gel-mcp/blob/main/src/gel_mcp/static/src/gel-python-auto.md), [typescript](https://github.com/geldata/gel-mcp/blob/main/src/gel_mcp/static/src/gel-typescript-auto.md).

## Editor configuration variants

**Cursor** has multiple options. Docs for [MCP](https://docs.cursor.com/context/model-context-protocol), [prompts](https://docs.cursor.com/context/rules).

1. Project-wide: user stores MCP configuration in `.cursor/mcp.json` and prompt files in custom Markdown-ese in `.cursor/rules/*.mdc`.
1. **Note**: need to manually toggle the server in the settings.
2. Global automatic: user clicks an `Add to Cursor` button in the readme, and that adds the MCP server to their editor globally.
3. Global prompt: user can manually add global rules using a clicky interface.

**Zed** limits programmatic ways to do things. Docs for [MCP](https://zed.dev/docs/ai/mcp), [prompts](https://zed.dev/docs/ai/rules).

1. Project-wide: user stores the prompt in a `.rules` file at the root of the project. That file can contain links to other files using `/file path/to/file.md` syntax to create a nested document.
2. The MCP configuration can only be configured in the global `settings.json` using a semi-custom syntax.
3. Alternatively the prompt can be added globally and configured to auto-append via the Rules Library, which is a clicky interface. The MCP can be distributed via an extension, which Gel currently does not have (requires Treesitter grammar).

**Claude Code** offers a third way: Docs for [MCP](https://docs.anthropic.com/en/docs/claude-code/mcp), [prompts](https://docs.anthropic.com/en/docs/claude-code/memory).

1. User or the agent creates a `CLAUDE.md` file to store prompts. That file can contain links to other files. Similar to a `README.md`, there can be multiple of this file located in nested directories, which the agent is going to treat as "scoped" to the enclosing directory.
2. User puts `.mcp.json` file at the root to configure the MCP server with normal syntax. There's also a `claude` CLI command for that.

## CLI command outline

- `gel configure-agent`
- `gel configure-agent `: add / update configuration for the editor of choice.
- `gel configure-agent list`: show supported editors and their current setup status

1. Check if configuration for Gel / other tools already exists.
2. Create backups of files to be edited.
3. Download Gel's configuration files and merge with existing.
1. For Cursor: `.cursor/mcp.json` and `.cursor/rules/gel-*.mdc`.
2. For Claude Code: `CLAUDE.md` and `docs/gel-*.md`.
3. For Zed: `.rules` and `docs/gel-*.md`
4. Print instructions where it's impossible to configure things programmatically.
1. For Cursor: toggle the server in the GUI.
2. For Zed: copy-pasteable snippet to put in `settings.json`, instruction for how to create auto-appendable prompt in the GUI.

Optionally offer an interactive mode to force the user to read the friendly manual and guide them to correct configuration.