chore(skills): add branch guard to pr-review skill

Reviews must run from main. Added step 0 hard stop if current branch
is not main, preventing repeated misuse from team branches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/pr-review/SKILL.md

@@ -16,11 +16,20 @@ on the branch type. All reviewers must approve for a clean review.
 
 ## Workflow
 
-### 1. Determine the branch
+### 0. Branch guard — MUST be on `main`
 
-If the user provided a branch name as argument, use it. Otherwise use the
-current branch (`git branch --show-current`). If on `main`, ask the user
-which branch to review.
+```bash
+git branch --show-current
+```
+
+If the current branch is **not `main`**, stop immediately and tell the user:
+"PR reviews must be run from the `main` worktree. Switch to `main` first."
+Do NOT proceed with the review from a team branch.
+
+### 1. Determine the branch to review
+
+If the user provided a branch name as argument, use it. Otherwise list open
+PRs and ask the user which branch to review.
 
 To list open PRs on Gitea:
 ```bash