docs(command-development): add SlashCommand tool coverage (#56)

## Summary

- Add comprehensive documentation for how Claude programmatically
invokes slash commands via the SlashCommand tool
- Create new reference file covering visibility requirements, character
budget, permission rules, and best practices
- Update skill description with trigger phrases for discoverability

## Problem

Fixes #48

The command-development skill documents the `disable-model-invocation`
frontmatter field but doesn't explain its relationship to the
SlashCommand tool. Users need to understand:

- What the SlashCommand tool is and when Claude uses it
- Why commands need `description` for visibility
- How the character budget (15,000 default) works
- Permission rule patterns for controlling programmatic invocation

## Solution

Created `references/slashcommand-tool.md` with complete coverage:

1. **SlashCommand Tool Overview** - What it does and when Claude uses it
2. **Visibility Requirements** - Why `description` field is required
3. **Character Budget** - 15,000 default,
`SLASH_COMMAND_TOOL_CHAR_BUDGET` env var
4. **Permission Rules** - Exact match, prefix match, deny patterns
5. **disable-model-invocation Context** - When to use vs permission
rules
6. **Integration Patterns** - Commands designed for programmatic vs
manual use
7. **Troubleshooting** - Common issues and solutions
8. **Best Practices** - For command authors and users

Updated SKILL.md:
- Added trigger phrases: "SlashCommand tool", "programmatic command
invocation", "disable-model-invocation", "prevent Claude from running
command"
- Added reference to new file in Additional Resources section

### Alternatives Considered

- **Expand frontmatter-reference.md** - Rejected because SlashCommand
tool deserves dedicated coverage beyond just frontmatter syntax
- **Add to SKILL.md directly** - Rejected to follow progressive
disclosure pattern (keep SKILL.md lean)

## Changes

-
`plugins/plugin-dev/skills/command-development/references/slashcommand-tool.md`
- New reference file (~420 lines)
- `plugins/plugin-dev/skills/command-development/SKILL.md` - Updated
description and Additional Resources

## Testing

- [x] markdownlint passes on both files
- [x] Progressive disclosure pattern followed (detailed content in
references/)
- [x] Trigger phrases align with issue requirements

---
🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: sjnims

plugins/plugin-dev/skills/command-development/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: command-development
-description: This skill should be used when the user asks to "create a slash command", "add a command", "write a custom command", "define command arguments", "use command frontmatter", "organize commands", "create command with file references", "interactive command", "use AskUserQuestion in command", or needs guidance on slash command structure, YAML frontmatter fields, dynamic arguments, bash execution in commands, user interaction patterns, or command development best practices for Claude Code.
+description: This skill should be used when the user asks to "create a slash command", "add a command", "write a custom command", "define command arguments", "use command frontmatter", "organize commands", "create command with file references", "interactive command", "use AskUserQuestion in command", "SlashCommand tool", "programmatic command invocation", "disable-model-invocation", "prevent Claude from running command", or needs guidance on slash command structure, YAML frontmatter fields, dynamic arguments, bash execution in commands, user interaction patterns, programmatic invocation control, or command development best practices for Claude Code.
 ---
 
 # Command Development for Claude Code
@@ -701,6 +701,7 @@ Commands should validate inputs and resources before processing:
 ## Additional Resources
 
 For detailed frontmatter field specifications, see `references/frontmatter-reference.md`.
+For SlashCommand tool, programmatic invocation, and permission configuration, see `references/slashcommand-tool.md`.
 For plugin-specific features and patterns, see `references/plugin-features-reference.md`.
 For plugin integration and validation patterns, see `references/plugin-integration.md`.
 For interactive user input patterns using AskUserQuestion, see `references/interactive-commands.md`.