Add AI Agent Selection Parameter to Dogfood Template

## Context

We now have multiple AI agents supporting Coder Tasks (Cursor, Codex, Claude, with Sourcegraph AMP coming soon). Currently, to dogfood different agents, we need to replicate the entire dogfood template for each agent.

## Proposal

Extend the existing dogfood template to include a parameter that allows users to select which AI agent to use for their workspace, similar to how we currently allow IDE selection.

## Requirements

- Add a `coder_parameter` for AI agent selection to the dogfood template
- Support at minimum: Cursor, Codex, Claude (with Sourcegraph AMP when available)
- Include necessary API key configuration for each supported agent
- Add conditional module logic to provision the selected agent
- Consider adding `display_on_task = true` property if needed for UX
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## UX Considerations

- Agent selection should be presented as part of the parameters during workspace creation

## Technical Notes

- Will require API keys for each model provider
- Need conditional module for each supported agent

## Benefits

- Enables dogfooding of all supported AI agents without template duplication
- Provides better testing coverage across different agent implementations
- Improves developer experience for internal teams