chore: install refactoring guardrails (Phase 0) [guardrail-change]

- scripts/check-loc.sh: LOC budget checker (500 LOC hard cap)
- .claude/rules/architecture.md: split triggers, patterns per language
- .claude/rules/loc-exceptions.txt: documented escape hatches
- AGENTS.python.md: FastAPI conventions (routes thin, service layer)
- AGENTS.go.md: Go/Gin conventions (handler ≤40 LOC)
- AGENTS.typescript.md: Next.js conventions (page.tsx ≤250 LOC, colocation)
- CLAUDE.md extended with guardrail section + commit markers

273 files currently exceed 500 LOC — to be addressed phase by phase.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/CLAUDE.md

@@ -256,3 +256,45 @@ ssh macmini "cd /Users/benjaminadmin/Projekte/breakpilot-lehrer && git push all
 | `website/app/admin/klausur-korrektur/` | Korrektur-Workspace |
 | `backend-lehrer/classroom_api.py` | Classroom Engine |
 | `backend-lehrer/state_engine_api.py` | State Engine |
+
+---
+
+## Code-Qualitaet Guardrails (NON-NEGOTIABLE)
+
+> Vollstaendige Details: `.claude/rules/architecture.md`
+> Ausnahmen: `.claude/rules/loc-exceptions.txt`
+
+### File Size Budget
+
+- **Hard Cap: 500 LOC** pro Datei
+- Wenn eine Aenderung eine Datei ueber 500 LOC bringen wuerde: **erst splitten, dann aendern**
+- Ausnahmen nur mit Begruendung in `loc-exceptions.txt` + `[guardrail-change]` Commit-Marker
+
+### Architektur
+
+- **Python:** Routes duenn → Business Logic in Services → Persistenz in Repositories
+- **Go:** Handler ≤40 LOC → Service-Layer → Repository-Pattern
+- **TypeScript/Next.js:** page.tsx duenn → Server Actions, Queries, Components auslagern
+- **Types:** Monolithische types.ts frueh splitten, types.ts + types/ Shadowing vermeiden
+
+### Workflow (bei jeder Aenderung)
+
+1. Datei lesen + LOC pruefen
+2. Wenn nahe am Budget → erst splitten
+3. Minimale kohaerente Aenderung
+4. Verifikation (Tests + Lint)
+5. Zusammenfassung: Was geaendert, was verifiziert, Restrisiko
+
+### Commit-Marker
+
+- `[migration-approved]` — Schema-/Migrations-Aenderungen
+- `[guardrail-change]` — Aenderungen an .claude/**, scripts/check-loc.sh
+- `[split-required]` — Aenderung beginnt mit Datei-Split
+- `[interface-change]` — Public API Contracts geaendert
+
+### LOC-Check ausfuehren
+
+```bash
+bash scripts/check-loc.sh --changed   # nur geaenderte Dateien
+bash scripts/check-loc.sh --all       # alle Dateien (zeigt alle Violations)
+```