feat(lifecycle): plain-language generation diffs (#82)

nomarchy-what-changed wraps nvd into "N added, M removed, K updated"
summaries. nomarchy-rebuild / nomarchy-home print full nvd and toast
the one-liner; System › What changed? offers toast + floating report.
checks.what-changed fixtures nvd via NOMARCHY_NVD.

Verified: V2 — flake check --no-build; checks.what-changed green;
local smoke on real HM generations.

README.md

@@ -187,6 +187,8 @@ nomarchy-pull      # nix flake update — refresh inputs (nomarchy, nixpkgs, …
 nomarchy-rebuild   # system rebuild against the CURRENT lock (sudo inside;
                    # BTRFS snapshot first when snapper is on; nvd diff after)
 nomarchy-home      # home-manager switch — desktop only, no sudo, no lock bump
+                   # (prints nvd + a "What changed" toast when the generation moved)
+nomarchy-what-changed  # plain-language last rebuild (system + desktop; --summary for one-liners)
 ```
 
 `~/.nomarchy` is **your machine flake** (`system.nix` / `home.nix`). Newer