feat: add Claude.md

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Repository Overview
+
+This is a NixOS infrastructure repository managing multiple hosts (servers and personal machines) using a modular Nix configuration approach with SOPS for secrets management and Bento for deployment.
+
+## Build and Test Commands
+
+```bash
+# Enter development shell (sets up MCP configs)
+nix-shell
+
+# Test configuration before deployment (required before PRs)
+./scripts/test-configuration <hostname>
+./scripts/test-configuration -v <hostname>  # with --show-trace
+
+# Edit encrypted secrets
+nix-shell -p sops --run 'sops hosts/<hostname>/secrets.yaml'
+
+# Update secrets keys after adding new age keys
+./scripts/update-secrets-keys
+
+# Format Nix files
+nix run nixpkgs#nixpkgs-fmt .
+
+# Compute hash for new packages
+nix hash to-sri --type sha256 $(nix-prefetch-url https://example.com/file.tar.gz)
+```
+
+## Architecture
+
+### Host Structure
+Each host in `hosts/<hostname>/` contains:
+- `configuration.nix` - Main entry point importing modules
+- `hardware-configuration.nix` - Machine-specific hardware config
+- `secrets.yaml` - SOPS-encrypted secrets
+- `modules/` - Host-specific service configurations
+- `fleet.nix` → symlink to root `fleet.nix` (SFTP user provisioning)
+- `utils/` → symlink to root `utils/` (shared modules)
+
+Current hosts: `fw` (firewall/router), `nb` (notebook), `web-arm`, `mail`, `amzebs-01`
+
+### Shared Components (`utils/`)
+- `modules/` - Reusable NixOS modules (nginx, sops, borgbackup, lego, promtail, etc.)
+- `overlays/` - Nixpkgs overlays
+- `pkgs/` - Custom package derivations
+- `bento.nix` - Deployment helper module
+
+### Secrets Management
+- SOPS with age encryption; keys defined in `.sops.yaml`
+- Each host has its own age key derived from SSH host key
+- Host secrets in `hosts/<hostname>/secrets.yaml`
+- Shared module secrets in `utils/modules/<module>/secrets.yaml`
+
+**IMPORTANT: Never modify secrets files directly.** Instead, tell the user which secrets need to be added and where, so they can edit the encrypted files themselves using:
+```bash
+nix-shell -p sops --run 'sops hosts/<hostname>/secrets.yaml'
+```
+
+### Deployment
+The Git runner handles deployment automatically when changes merge to main. A successful `./scripts/test-configuration <host>` dry-build is the gate before pushing.
+
+## Custom Packages
+
+When creating a new package in `utils/pkgs/`, always include an `update.sh` script to automate version updates. See `utils/pkgs/claude-code/update.sh` for the pattern:
+
+1. Fetch latest version from upstream (npm, GitHub, etc.)
+2. Update version string in `default.nix`
+3. Update source hash using `nix-prefetch-url`
+4. Update dependency hashes (e.g., `npmDepsHash`) by triggering a build with a fake hash
+5. Verify the final build succeeds
+
+Example structure:
+```
+utils/pkgs/<package-name>/
+├── default.nix
+├── update.sh          # Always include this
+└── (other files like patches, lock files)
+```
+
+## Conventions
+
+- Nix files: two-space indentation, lower kebab-case naming
+- Commits: Conventional Commits format (`fix:`, `feat:`, `chore:`), scope by host when relevant (`fix(mail):`)
+- Modules import via explicit paths, not wildcards
+- Comments explain non-obvious decisions (open ports, unusual service options)