feat: add debug mode to capture agent tool log

When debug: true, the review comment includes a collapsible section
showing which files the agent read, grep patterns used, etc.
Also prints agent log on failure for easier troubleshooting.

entrypoint.sh

@@ -16,6 +16,7 @@ export PI_REVIEW_PROMPT="${INPUT_REVIEW_PROMPT:-}"
 export PI_EXCLUDE="${INPUT_EXCLUDE_PATTERNS:-*.lock package-lock.json yarn.lock pnpm-lock.yaml *.min.js *.min.css *.map}"
 export PI_MAX_DIFF="${INPUT_MAX_DIFF_BYTES:-80000}"
 export PI_TOKEN="${INPUT_TOKEN}"
+export PI_DEBUG="${INPUT_DEBUG:-false}"
 
 # The calling repo is mounted at GITHUB_WORKSPACE by both GitHub and Gitea.
 # cd into it so git commands work against the right repo.