docs: add AGENTS.md for OpenCode session context

AGENTS.md

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+# AGENTS.md
+
+## Overview
+
+Nix flake managing 3 devices: Mac Mini, MacBook Air (both aarch64-darwin via nix-darwin), and a Windows PC via NixOS-WSL (x86_64-linux). Single user `imbytecat` everywhere.
+
+## Architecture
+
+```
+flake.nix
+├── darwinConfigurations.mac-mini    (aarch64-darwin)
+├── darwinConfigurations.macbook-air (aarch64-darwin)
+└── nixosConfigurations.wsl          (x86_64-linux)
+```
+
+- `lib/default.nix` — builders: `mkDarwin`, `mkNixos`, `mkHome`
+- `modules/shared/` — both platforms (nixpkgs config, overlays)
+- `modules/darwin/` — macOS: system preferences, homebrew (casks/brews/masApps), fonts
+- `modules/nixos/` — NixOS: base packages, docker, locale, user
+- `home/` — home-manager (shared across all hosts via `useGlobalPkgs`)
+- `overlays/` — custom packages (comment-checker)
+
+## Critical gotchas
+
+- **Determinate Nix on macOS**: `nix.enable = false` in `modules/shared/nix.nix` for darwin. Determinate manages the nix daemon; nix-darwin must not. `nix.settings` only applies on NixOS.
+- **catppuccin.nvim disabled**: `catppuccin.nvim.enable = false` in `home/theme.nix` due to `catppuccin.lib.detect_integrations` require check failure in nixpkgs. Re-test periodically.
+- **catppuccin module name**: Uses `catppuccin.homeModules.catppuccin` (not the old `homeManagerModules`).
+- **Homebrew tap casks**: Casks from taps need full path in the casks list (e.g. `"goooler/repo/fl-clash"`), not just the short name.
+- **`onActivation.cleanup = "zap"`**: Any brew formula/cask NOT declared in `modules/darwin/default.nix` WILL be removed on rebuild. Be comprehensive.
+- **Repo location**: The repo lives at `~/Developer/nix-config`, NOT `~/.config/nix-config`. The `rebuild` fish abbreviation references `~/.config/nix-config` — symlink or update if needed.
+
+## Commands
+
+```bash
+# Validate (eval only, fast)
+nix build .#darwinConfigurations.mac-mini.system --dry-run
+
+# Validate (actual build, catches runtime failures like require checks)
+nix build .#darwinConfigurations.mac-mini.system
+
+# First-time bootstrap (nix-darwin not yet installed)
+sudo nix run nix-darwin -- switch --flake .#mac-mini
+
+# Daily rebuild (after first bootstrap)
+sudo darwin-rebuild switch --flake .#mac-mini
+
+# WSL rebuild
+sudo nixos-rebuild switch --flake .#wsl
+
+# Update all inputs
+nix flake update
+```
+
+## Shell
+
+Fish (not zsh). All tool integrations use `enableFishIntegration`. Key files:
+- `home/shell/fish.nix` — abbreviations, interactiveShellInit
+- `home/shell/tools.nix` — fzf, atuin, zoxide (`--cmd cd`), direnv, bat, eza, yazi, btop
+
+## Home Manager option API (current)
+
+These options were renamed in recent home-manager; use the new names:
+- `programs.git.settings.user.{name,email}` (not `userName`/`userEmail`)
+- `programs.git.settings.*` (not `extraConfig`)
+- `programs.delta.{enable,options}` (not `programs.git.delta.*`)
+- `programs.delta.enableGitIntegration = true` (must be explicit)
+- `programs.ssh.matchBlocks."*".addKeysToAgent` (not top-level `addKeysToAgent`)
+- `programs.ssh.enableDefaultConfig = false` (set explicitly)
+
+## Nix LSP
+
+Uses `nixd` (not `nil`). nixd provides nixpkgs/option completion; nil does not. Installed via `home/dev/languages.nix`.
+
+## Formatter
+
+`nixfmt` (was `nixfmt-rfc-style`, now unified). Run: `nixfmt <file.nix>`