feat: add optional prompt argument to /close-issue slash command

## Summary
Enhance the `/close-issue` slash command to accept an optional prompt argument after the GitHub issue number/URL, allowing users to provide additional context or specific instructions that get concatenated to the end of the standard prompt.

## Current State
```bash
/close-issue 123
/close-issue https://github.com/owner/repo/issues/123
```

## Proposed Enhancement
```bash
/close-issue 123 "focus on performance optimization"
/close-issue https://github.com/owner/repo/issues/123 "use the new MCP tools"
```

## Implementation Details
- The additional string would be concatenated at the end of the existing prompt
- Simple string concatenation, no complex parsing needed
- Maintains backward compatibility (optional argument)

## Rationale
- **OSE perspective**: Allows human to steer at the macro level without micro-managing
- **DevEx improvement**: Reduces need for follow-up clarifications
- **Efficiency**: One command with context vs command + explanation

## Principle Alignment
- **OSE**: Operate at higher altitude by providing strategic direction upfront
- **Developer Experience**: Reduces friction in the human-AI collaboration

Locate the implementation of the /close-issue slash command and trace how it parses the issue number or URL and builds the standard prompt. Add support for an optional trailing prompt while preserving existing invocations, then verify both numbered and URL-based examples retain their current behavior and include the extra context.

Written by the indexing model from the issue text.