Implement agent version tracking to diagnose deployment issues:
- Add get_agent_hash() method to extract Nix store hash from executable path
- Collect system_agent_hash metric in NixOS collector
- Display "Agent Hash" in system panel under NixOS section
- Update metric filtering to include agent hash
This helps identify which version of the agent is actually running
when troubleshooting deployment or metric collection issues.
- Strip codename part (e.g., '(Warbler)') from nixos-version output
- Display clean version format: '25.05.20251004.3bcc93c'
- Simplify parsing to use raw nixos-version output as requested
- Change from showing version to build format: 'hash dd/mm/yy H:M:S'
- Parse nixos-version output to extract short hash and format date
- Update system widget to display 'Build:' instead of 'Version:'
- Remove version/build_date fields in favor of single build string
- Follow TODO.md specification for NixOS section layout
- Create NixOS collector for version and active users detection
- Add SystemWidget combining all system information in TODO.md layout
- Replace separate CPU/Memory widgets with unified system display
- Add tree structure for storage with drive temperature/wear info
- Support NixOS version, active users, load averages, memory usage
- Follow exact decimal formatting from specification
