chore: add multi-tool triage skill

.github/copilot-instructions.md

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+# Triage Findings
+
+When the task is to triage review findings, follow this workflow:
+
+1. Merge all findings into one list.
+2. Remove duplicates.
+3. Sort by severity: `critical` -> `warning` -> `info`.
+4. Renumber from 1 after sorting.
+5. Fix real issues with the smallest safe change.
+6. Add false positives to `.gitea/ai-review/exclusions.json`.
+7. Add or update tests when behavior changes.
+8. Re-check the issue after each fix.
+
+Use the repo-local `triage-findings` skill for the same workflow when running in Codex.