feat: add explicit model specification to workflow commands (#140)

## Summary
- Add `model: sonnet` frontmatter field to `/plugin-dev:create-plugin`
and `/plugin-dev:create-marketplace` commands
- Ensures consistent workflow quality regardless of user's current
conversation model

## Problem
Fixes #137

Complex 8-phase workflows inherit model from conversation context,
causing inconsistent quality. Users on `haiku` would get different
results than those on `opus`.

## Solution
Explicitly specify `model: sonnet` for workflow commands - a good
balance of capability and cost for complex multi-step workflows.

### Alternatives Considered
- **Keep inheriting from conversation** (original behavior): Allows user
cost/quality control but risks inconsistent results
- **Use `opus`**: Maximum quality but higher cost for 8-phase workflows
- **Use `haiku`**: Faster but may not handle complex design phases well

## Changes
- `plugins/plugin-dev/commands/create-plugin.md`: Added `model: sonnet`
to frontmatter
- `plugins/plugin-dev/commands/create-marketplace.md`: Added `model:
sonnet` to frontmatter

## Testing
- [x] Linting passes
- [x] No structural changes to command content

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

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

Author: sjnims

plugins/plugin-dev/commands/create-marketplace.md

@@ -2,6 +2,7 @@
 description: Create plugin marketplaces with guided workflow
 argument-hint: [marketplace-description]
 allowed-tools: Read, Write, Edit, Grep, Glob, Bash(mkdir:*), Bash(git init:*), TodoWrite, AskUserQuestion, Skill, Task
+model: sonnet
 disable-model-invocation: true
 ---
 

plugins/plugin-dev/commands/create-plugin.md

@@ -2,6 +2,7 @@
 description: Create plugins with guided 8-phase workflow
 argument-hint: [plugin-description]
 allowed-tools: Read, Write, Edit, Grep, Glob, Bash(mkdir:*), Bash(git init:*), TodoWrite, AskUserQuestion, Skill, Task
+model: sonnet
 disable-model-invocation: true
 ---