Make AI PR Generator modular to support multiple AI models (Claude, Codex)

## Description

Currently, the AI PR Generator (PR #229) is designed specifically for Claude Code. We should make it modular to support multiple AI models, including Codex, allowing users to choose which AI model to use based on their preferences or available API keys.

## Current Behavior

- The PR generator is tightly coupled to Claude Code
- Only supports Claude's API and authentication methods
- Configuration is specific to Claude's command-line parameters

## Expected Behavior

- Support multiple AI models through a modular architecture
- Allow selection of AI model via workflow input parameter or label prefix
- Handle different authentication methods for each supported model
- Maintain consistent PR generation workflow regardless of the AI model used

## Implementation Details

### Proposed Architecture

1. **Model Abstraction Layer**:
- Create a common interface for AI model interactions
- Implement model-specific adapters (Claude, Codex, etc.)

2. **Configuration Management**:
- Support model-specific configuration and secrets
- Allow different prompt formats optimized for each model

3. **Model Selection**:
- Add a parameter to select the model: `ai-model: [claude|codex]`
- Support label-based selection (e.g., `claude-generate`, `codex-generate`)

### Example Workflow Input

```yaml
workflow_dispatch:
inputs:
issue_number:
description: 'Issue number to process'
required: true
type: number
ai_model:
description: 'AI model to use'
required: true
default: 'claude'
type: choice
options:
- claude
- codex
```

### Required Secrets

- `ANTHROPIC_API_KEY` - For Claude
- `OPENAI_API_KEY` - For Codex/OpenAI models
- Additional keys as needed for other models

## References

- Current implementation in PR #229: https://github.com/atxtechbro/dotfiles/pull/229
- Claude Code documentation: https://docs.anthropic.com/claude/docs/claude-code-sdk
- OpenAI Codex documentation: https://platform.openai.com/docs/guides/code

## Related Issues

- #228 - Original issue for Claude Code PR Generator

Start by reading the current implementation in PR #229 and the context from issue #228. Trace how the workflow dispatch input, Claude-specific parameters, authentication, and PR generation are connected. Done means the workflow can select Claude or Codex, use the corresponding secret and prompt configuration, and preserve the same PR-generation flow.

Written by the indexing model from the issue text.