docs: establish domain model, MVI spec, and environment reference

Capture the design work for the NixOS migration before any implementation:

- .claude/CONTEXT.md: domain glossary (Host, Module, Skeleton, Auto-loader,
  Enable convention, unstable/stable overlay)
- .claude/adr/0001-sops-nix-for-secrets.md: secrets tooling decision
- .claude/spec/laptop-mvi.md: frozen minimum-viable-install spec for neogaia

- reference/: read-only snapshot of the current CachyOS configs (secrets and
  state excluded), plus ENVIRONMENT.md profiling the live environment to guide
  replication

.claude/CONTEXT.md

@@ -0,0 +1,33 @@
+# NixOS Dotfiles
+
+A single flake that builds every machine the user owns — laptop, desktop, and three servers — from one shared, modular configuration.
+
+## Language
+
+**Host**:
+One physical machine the flake builds a NixOS configuration for. Each Host has a directory under `hosts/` holding its machine-specific `hardware-configuration.nix` and its choice of enabled Modules.
+_Avoid_: machine, node, system, box
+
+**Module**:
+A single `.nix` feature file under `modules/` that declares an `enable` option and the configuration it turns on. Every Module is always imported but stays inert until a Host enables it.
+_Avoid_: component, package, plugin
+
+**Skeleton**:
+The flake's plumbing — the Auto-loader, the helper lib, the flake inputs/overlays, and the shared base config — as distinct from the Modules that sit on top of it.
+_Avoid_: framework, core, base, scaffolding
+
+**Auto-loader**:
+The lib code that recursively discovers and imports every Module under `modules/` (and every Host under `hosts/`) so new files wire themselves in without manual `imports` edits.
+_Avoid_: loader, importer, scanner
+
+**Enable convention**:
+The rule that every Module is imported unconditionally and guards its own body with `mkIf config.modules.<path>.enable`, so a Host reads as a checklist of `enable = true` flags.
+_Avoid_: feature flag, toggle, opt-in
+
+**unstable overlay**:
+The overlay exposing `nixpkgs-unstable` packages as `unstable.<name>`, used to pull an individual package fresher than the `nixos-unstable` base.
+_Avoid_: bleeding-edge, latest
+
+**stable overlay**:
+The overlay exposing the latest stable release (`nixos-25.05`) as `stable.<name>`, used to pin an individual package to the rock-solid release from the `nixos-unstable` base.
+_Avoid_: LTS, release channel