chore: add multi-tool triage skill

GEMINI.md

@@ -0,0 +1,14 @@
+# Triage Findings
+
+Use the triage-finding workflow for review issue lists:
+
+1. Merge findings into one list.
+2. Remove duplicates.
+3. Sort by severity: `critical` -> `warning` -> `info`.
+4. Renumber from 1.
+5. Fix real issues with the smallest safe change.
+6. Put false positives into `.gitea/ai-review/exclusions.json`.
+7. Add or update tests when behavior changes.
+8. Re-check after each fix.
+
+The reusable skill lives in `.gemini/skills/triage-findings/SKILL.md`.