Add T490 hardware configuration and docs
- modules/t490.nix: TLP power management, CPU throttling (throttled), S3 sleep,
remote Nix builders (cluster), fwupd, fprintd, fprintd, Syncthing, Tailscale,
Intel VA-API, TrackPoint sensitivity, PipeWire defaults
- flake.nix: add nixos-hardware input, import T490 module + nixos-hardware
- docs/T490-HARDWARE.md: WiFi variants, graphics, audio, fingerprint, sleep guide
- docs/T490-BATTERY.md: TLP config, threshold strategy, real-world battery numbers
- docs/T490-OFFLOADING.md: local vs cluster split, remote builder setup
- MIGRATION.md: add T490 post-install steps (fwupd, fprint, remote build key)
- README.md: reflect new module and docs structure
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