ai(claude[commands]) Namespace multi-model commands under multi-model directory

why: Group related review commands under a shared namespace for
discoverability (/multi-model:review, /multi-model:solve).
what:
- Move multi-model-review.md → multi-model/review.md
- Move solve-review.md → multi-model/solve.md
- Update cross-reference from /multi-model-review to /multi-model:review

Author: tony

.claude/commands/multi-model-review.md .claude/commands/multi-model/review.md.claude/commands/multi-model-review.md renamed to .claude/commands/multi-model/review.md

@@ -15,7 +15,7 @@ Process multi-model code review findings from the conversation context. Validate
 
 **Actions**:
 
-1. **Locate the review report** in the conversation context (output from `/multi-model-review`)
+1. **Locate the review report** in the conversation context (output from `/multi-model:review`)
 
 2. **Extract each finding** into a numbered list with:
    - **Consensus level**: how many reviewers flagged it (3, 2, or 1)