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>

AGENTS.go.md

@@ -0,0 +1,36 @@
+# AGENTS.go.md — Go/Gin Konventionen
+
+## Architektur
+
+- `handlers/`: HTTP Transport nur — Decode, Validate, Call Service, Encode Response
+- `service/` oder `usecase/`: Business Logic
+- `repo/`: Storage/Integration
+- `model/` oder `domain/`: Domain Entities
+- `tests/`: Table-driven Tests bevorzugen
+
+## Regeln
+
+1. Handler ≤40 LOC — nur Decode → Service → Encode
+2. Business Logic NICHT in Handlers verstecken
+3. Grosse Handler nach Resource/Verb splitten
+4. Request/Response DTOs nah am Transport halten
+5. Interfaces nur an echten Boundaries (nicht ueberall fuer Mocks)
+6. Keine Giant-Utility-Dateien
+7. Generated Files nicht manuell editieren
+
+## Split-Trigger
+
+- Handler-Datei ueberschreitet 400-500 LOC
+- Unrelated Endpoints zusammengruppiert
+- Encoding/Decoding dominiert die Handler-Datei
+- Service-Logik und Transport-Logik gemischt
+
+## Verifikation
+
+```bash
+gofmt -l . | grep -q . && exit 1
+go vet ./...
+golangci-lint run --timeout=5m
+go test -race ./...
+go build ./...
+```