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

reference/home/.config/shelly/config.json

@@ -0,0 +1,56 @@
+{
+  "FileSizeDisplay": "Megabytes",
+  "DefaultExecution": "UpgradeAll",
+  "ParallelDownloadCount": 10,
+  "Culture": "",
+  "AurEnabled": true,
+  "ShellySearchEnabled": false,
+  "AurWarningConfirmed": true,
+  "FlatPackEnabled": true,
+  "WindowWidth": 1021,
+  "WindowHeight": 600,
+  "DefaultView": "HomeScreen",
+  "UseOldMenu": false,
+  "TrayEnabled": true,
+  "TrayCheckIntervalHours": 72,
+  "NoConfirm": false,
+  "NewInstall": false,
+  "CurrentVersion": "2.4.1.2",
+  "UseWeeklySchedule": false,
+  "DaysOfWeek": [],
+  "Time": null,
+  "ShellyIconsEnabled": true,
+  "AppImageEnabled": true,
+  "NewInstallInitSettings": true,
+  "UseSymbolicTray": true,
+  "RemoveCache": false,
+  "TrayIconPath": "",
+  "TrayUpdatesIconPath": "",
+  "DefaultPageDropDown": "Packages",
+  "RecommendedEnabled": true,
+  "ProgressBarStyle": "Blocks",
+  "ProgressBarFps": 7,
+  "ProgressBarWidth": 24,
+  "OutputMode": "singlepane",
+  "SinglePaneMaxStickies": 6,
+  "TrayAutoStart": false,
+  "PackageDowngradeEnabled": false,
+  "PackageManagementCascadeDelete": true,
+  "PackageManagementRemoveConfigs": false,
+  "PackageManagementRemoveOptionalDeps": true,
+  "PackageManagementShowHidden": false,
+  "PackageInstallUpgrade": false,
+  "PackageInstallShowHidden": false,
+  "PackageUpdateShowHidden": false,
+  "AurInstallUseChroot": false,
+  "AurInstallRunChecks": false,
+  "AurRemoveCascadeDelete": true,
+  "AurRemoveShowHidden": false,
+  "AurUpdateRunChecks": false,
+  "AurUpdateShowHidden": false,
+  "AppImageInstallPath": "",
+  "StarFishEnabled": false,
+  "PackageInstallView": "Grid",
+  "PackageUpdateView": "Grid",
+  "PackageManageView": "Grid"
+}