Add CLAUDE.md with project conventions and dev setup

Provides persistent context for Claude Code sessions: Rust stack,
multi-model AI, Vault integration, and self-hosted deployment conventions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: haasonsaas

CLAUDE.md

@@ -0,0 +1,29 @@
+# DiffScope - AI Code Review Engine
+
+## Quick Reference
+```bash
+cargo build               # Build
+cargo test                # Run tests
+cargo run -- --help       # CLI usage
+```
+
+## Architecture
+- **Language**: Rust
+- **Database**: PostgreSQL (via sqlx with compile-time checked queries)
+- **AI providers**: Multi-model support (Anthropic primary, OpenAI, OpenRouter)
+- **Deployment**: Docker → k3s via Helm chart in `charts/`
+- **GitHub integration**: GitHub App auth (not PATs)
+
+## Key Directories
+- `src/` — Core analysis engine, CLI, API
+- `charts/` — Helm chart for k8s deployment
+- `migrations/` — PostgreSQL migrations (sqlx)
+- `eval/` — Evaluation and benchmarking
+- `examples/` — Usage examples
+
+## Conventions
+- Use frontier models for reviews — never default to smaller models
+- Vault integration for secrets management (HashiCorp Vault)
+- GitHub App authentication over personal access tokens
+- Wide events for observability (OpenTelemetry-compatible)
+- Self-hosted first: Ollama/vLLM/LM Studio should be first-class providers