docs: add README.md for this project

README.md

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+# nix-darwin
+
+Declarative macOS setup for host `fenrir` using `nix-darwin`, `home-manager`,
+`nix-homebrew`, and `sops-nix`.
+
+## What This Repo Manages
+
+- System-level macOS configuration (`modules/`)
+- User-level tooling and shell/editor setup (`home/`)
+- Declarative Homebrew taps/apps/casks
+- Encrypted secrets via SOPS (`secrets/` + `.sops.yaml`)
+
+## Prerequisites
+
+- macOS on Apple Silicon (`aarch64-darwin`)
+- Nix with flakes (`nix-command` + `flakes`)
+- `just` (command runner)
+- SOPS age key at:
+
+```bash
+~/.config/sops/age/keys.txt
+```
+
+## Repository Layout
+
+- `flake.nix`: flake inputs/outputs and `darwinConfigurations`
+- `Justfile`: daily commands (`darwin`, `darwin-debug`, `fmt`, `up`, `gc`, etc.)
+- `modules/`: system modules (`nix-core.nix`, `system.nix`, `apps.nix`,
+  `host-users.nix`)
+- `home/`: Home Manager modules (shell, git, nvf, terminal, tools)
+- `secrets/`: encrypted secret files (`poby.yaml`)
+
+## Common Commands
+
+```bash
+# List available tasks
+just
+
+# Build and switch for a host
+just darwin $(hostname)
+
+# Build and switch with full trace
+just darwin-debug $(hostname)
+
+# Format Nix files (example: whole repo)
+just fmt .
+
+# Update all flake inputs
+just up
+
+# Update one input
+just upp nixpkgs-darwin
+
+# Inspect system profile history
+just history
+
+# Clean old generations / garbage collect
+just clean
+just gc
+```
+
+## Secrets
+
+- Secrets are encrypted in `secrets/*.yaml`.
+- `.sops.yaml` enforces age-based encryption rules.
+- Home Manager reads secrets from `secrets/poby.yaml` and exposes:
+  - `github_ssh_key`
+  - `github_cli_token`
+
+## Customization Notes
+
+- Update `hostname`, `username`, and `useremail` in `flake.nix` for your
+  machine.
+- Add new system behavior in `modules/*.nix`.
+- Add user tooling in `home/*.nix` and import it from `home/default.nix`.
+
+## Troubleshooting
+
+- Use `just darwin-debug <hostname>` for verbose evaluation/build output.
+- If a build succeeds but behavior is stale, re-run switch and verify active
+  host/config values.