refactor: modularize nvim config into lua/config + lua/plugins

- Extract all options, keymaps, and autocommands into lua/config/
- Move every plugin spec into its own file under lua/plugins/
- Add custom inline orng colorscheme (dark + light) synced to macOS appearance
- Consolidate image.nvim into pdf-viewer.lua (fix duplicate spec)
- Fix image.nvim processor: magick -> magick_cli
- Remove conflicting nvim-autopairs (kept mini.pairs)
- Fix broken <C-f> keymap (was calling non-existent builtin cmd)
- Delete lua/kickstart/ and lua/custom/ folders
- Add colors/orng.vim + colors/orng-light.vim stubs for mid-session switching
- init.lua reduced from 1078 lines to 49 lines

lua/plugins/lint.lua

@@ -0,0 +1,25 @@
+-- lua/plugins/lint.lua
+-- Async linting via nvim-lint
+
+return {
+  'mfussenegger/nvim-lint',
+  event = { 'BufReadPre', 'BufNewFile' },
+  config = function()
+    local lint = require 'lint'
+
+    lint.linters_by_ft = {
+      markdown = { 'markdownlint' },
+    }
+
+    local augroup = vim.api.nvim_create_augroup('user-lint', { clear = true })
+    vim.api.nvim_create_autocmd({ 'BufEnter', 'BufWritePost', 'InsertLeave' }, {
+      group    = augroup,
+      callback = function()
+        -- Only lint modifiable buffers to avoid noise in readonly pop-ups
+        if vim.bo.modifiable then
+          lint.try_lint()
+        end
+      end,
+    })
+  end,
+}