github: fix claude-code-review running in tag mode for issue_comment

Without a prompt the action auto-detected mode: tag instead of agent
when triggered via issue_comment, resulting in no review being posted.
Remove the prompt so the action can correctly use its built-in agent
mode with full PR context.

Fixes: c0e75c9de6 ("github: add Claude code review workflow triggered by /claude comment")
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

.github/workflows/claude-code-review.yml

@@ -31,6 +31,3 @@ jobs:
           anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
           github_token: ${{ secrets.GITHUB_TOKEN }}
           trigger_phrase: "/claude"
-          prompt: >-
-            Review this PR for an OpenWrt embedded Linux project,
-            focusing on correctness, security, and coding conventions.