feat(neovim): replace nixvim with programs.neovim (#336)

nixvim has been disabled for a long time (programs.nixvim.enable = false), so
drop the flake input (and prune flake.lock). Port the settings it carried to
Home Manager's programs.neovim: opts and per-plugin setup become an initLua
block plus a plugin list. The catppuccin module already provides the
colorscheme, vi stays unmapped, and the plain system neovim package is
untouched so sudo/nvim --clean keep using it.

flake.lock

@@ -260,27 +260,6 @@
         "type": "github"
       }
     },
-    "flake-parts_4": {
-      "inputs": {
-        "nixpkgs-lib": [
-          "nixvim",
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1788450739,
-        "narHash": "sha256-glZLQlzIn1fXH6PazR2iUmTo7kzzyYSshrWhLS9TqCU=",
-        "owner": "hercules-ci",
-        "repo": "flake-parts",
-        "rev": "31729ca8cbdb4fa927b34e5f4353e6a83f39e993",
-        "type": "github"
-      },
-      "original": {
-        "owner": "hercules-ci",
-        "repo": "flake-parts",
-        "type": "github"
-      }
-    },
     "git-hooks": {
       "inputs": {
         "flake-compat": [
@@ -748,29 +727,6 @@
         "type": "github"
       }
     },
-    "nixvim": {
-      "inputs": {
-        "flake-parts": "flake-parts_4",
-        "nixpkgs": [
-          "nixpkgs"
-        ],
-        "systems": "systems_3"
-      },
-      "locked": {
-        "lastModified": 1789311332,
-        "narHash": "sha256-JNsVNLux4emA7/BeZYzzAKnBaPnkatpaf/Fwf7PQT4E=",
-        "owner": "nix-community",
-        "repo": "nixvim",
-        "rev": "0043fc97e267fee06ac0d858399ff6fac4242a92",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-community",
-        "ref": "nixos-26.05",
-        "repo": "nixvim",
-        "type": "github"
-      }
-    },
     "nu_scripts": {
       "flake": false,
       "locked": {
@@ -910,7 +866,6 @@
         "nixpkgs-master": "nixpkgs-master",
         "nixpkgs-patched": "nixpkgs-patched",
         "nixpkgs-stable": "nixpkgs-stable",
-        "nixvim": "nixvim",
         "nu_scripts": "nu_scripts",
         "nur-ryan4yin": "nur-ryan4yin",
         "pre-commit-hooks": "pre-commit-hooks",
@@ -986,21 +941,6 @@
         "type": "github"
       }
     },
-    "systems_3": {
-      "locked": {
-        "lastModified": 1681028828,
-        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
-        "owner": "nix-systems",
-        "repo": "default",
-        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-systems",
-        "repo": "default",
-        "type": "github"
-      }
-    },
     "treefmt-nix": {
       "inputs": {
         "nixpkgs": [