feat: create modular user configurations

User Accounts:
✅ geir - Primary user (development, desktop, multimedia)
✅ sma - Admin user (Diziet Sma, system administration)
✅ common.nix - Shared user settings and security

Key Features:
🔧 Culture character naming (sma = Diziet Sma, SC agent)
🔒 Security-focused admin account (SSH keys only, passwordless sudo)
🛠️ Development-focused primary user (containers, virtualization, creative tools)
📦 Modern CLI tools and shell enhancements
🎯 Role-based package selection and group memberships

Security Model:
- SSH key authentication for admin users
- Separate admin and daily-use accounts
- Principle of least privilege
- No root login allowed

Integration:
- Container runtime access (podman, incus)
- Virtualization management (libvirt, virt-manager)
- Development workflow (git, editors, languages)
- Desktop environments (GNOME, Cosmic, Sway)

Ready for machine-specific deployment across home lab infrastructure.

plan.md

@@ -312,9 +312,11 @@ Home-lab/
   - **Infrastructure**: Container-focused (Podman), PostgreSQL database
   - **Integration**: Central Git hosting for all home lab projects
 - [ ] Plan for additional users across machines:
-  - Service accounts for automation
-  - Admin accounts for management
-  - Guest accounts for temporary access
+  - [x] **geir** - Primary user (development, desktop, daily use)
+  - [x] **sma** - Admin user (Diziet Sma, system administration, security oversight)
+  - [ ] Service accounts for automation (forgejo-admin, backup-agent)
+  - [ ] Guest accounts for temporary access
+  - [x] Culture character naming convention established
 - [ ] Network infrastructure planning
 - [ ] Consider hardware requirements for future expansion