feat(home): add declarative Neovim via nixvim

New `nixvim` flake input (release branch, follows nixpkgs) drives the whole
editor from `modules/nvim.nix`, wired into `homeModules.default`. Minimal LSP
baseline (`nixd`, `bashls`) exposed as `local.nvim.lspServers`; blink-cmp,
Telescope, neo-tree, lualine, treesitter, colorizer, gitsigns, toggleterm, and a
catppuccin theme. which-key groups plus `<leader>?`/`<leader>sk` give an
in-editor keybind cheatsheet; per-directory session persistence with
`sessionoptions` + a `PersistenceSavePre` neo-tree close for clean layouts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

flake.lock

@@ -1,5 +1,26 @@
 {
   "nodes": {
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "nixvim",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1778716662,
+        "narHash": "sha256-m1Yf0wZ8j1OHjTc2UwHwyQRSnNeSgLJOd7q5Y45hzi4=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "f7c1a2d347e4c52d5fb8d10cb4d94b5884e546fb",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
     "home-manager": {
       "inputs": {
         "nixpkgs": [
@@ -37,10 +58,34 @@
         "type": "github"
       }
     },
+    "nixvim": {
+      "inputs": {
+        "flake-parts": "flake-parts",
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1782919967,
+        "narHash": "sha256-pRwjfB5HQJ3m8J8bOR43pPHtHI7VUJSqwLA3P06cOY0=",
+        "owner": "nix-community",
+        "repo": "nixvim",
+        "rev": "667c8471f4a0fb24d702d1a61af8609f1a5f1ba6",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "ref": "nixos-26.05",
+        "repo": "nixvim",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
         "home-manager": "home-manager",
         "nixpkgs": "nixpkgs",
+        "nixvim": "nixvim",
         "rust-overlay": "rust-overlay"
       }
     },
@@ -63,6 +108,21 @@
         "repo": "rust-overlay",
         "type": "github"
       }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
     }
   },
   "root": "root",