refactor: Complete modular migration from kickstart.nvim

Major architectural overhaul to transform the flat kickstart.nvim structure
  into a maintainable, modular configuration while preserving upstream sync capability.

  ## Structure Changes
  - Migrated from flat `lua/custom/` to organized `lua/core/` and `lua/plugins/`
  - Separated plugin specs from configs: `lua/plugins/spec/` and `lua/plugins/config/`
  - Complex configs (LSP, Debug) now use directory structure with sub-modules:
    - `lsp/init.lua`, `lsp/servers.lua`, `lsp/keymaps.lua`
    - `debug/init.lua`, `debug/adapters.lua`, `debug/keymaps.lua`

  ## Core Improvements
  - Created dedicated core modules: options, keymaps, autocmds, bootstrap, health
  - Added comprehensive health check (`lua/core/health.lua`) for diagnostics
  - Simplified init.lua to just orchestrate module loading
  - Better separation of concerns throughout

  ## Plugin Updates
  - Fixed Blink.cmp configuration (removed invalid fuzzy options)
  - Integrated Copilot with Blink.cmp for unified completion experience
  - Added autopairs and indent-line from kickstart examples
  - Optimized for Nix development environments (removed venv assumptions)

  ## Documentation
  - Updated README with modular structure and kickstart sync instructions
  - Created comprehensive KEYBIND_ANALYSIS.md with all mappings
  - Added modular.txt help documentation
  - Created TODO_TEST.md checklist for testing

  ## Benefits
  - Easier to maintain and extend
  - Clean separation allows upstream kickstart merges without conflicts
  - Scalable architecture for adding new languages/tools
  - Better code organization and discoverability

  All kickstart functionality preserved while gaining modularity and maintainability.

lua/plugins/spec/treesitter.lua

@@ -0,0 +1,45 @@
+-- Treesitter - Syntax highlighting and text objects
+return {
+  'nvim-treesitter/nvim-treesitter',
+  event = { 'BufReadPost', 'BufNewFile' },
+  build = ':TSUpdate',
+  main = 'nvim-treesitter.configs',
+  opts = {
+    ensure_installed = {
+      'bash',
+      'c',
+      'cmake',
+      'cpp',
+      'diff',
+      'html',
+      'lua',
+      'luadoc',
+      'make',
+      'markdown',
+      'markdown_inline',
+      'nix',
+      'python',
+      'query',
+      'vim',
+      'vimdoc',
+      'yaml',
+    },
+    auto_install = true,
+    highlight = {
+      enable = true,
+      additional_vim_regex_highlighting = false,
+    },
+    indent = {
+      enable = true,
+    },
+    incremental_selection = {
+      enable = true,
+      keymaps = {
+        init_selection = '<c-space>',
+        node_incremental = '<c-space>',
+        scope_incremental = false,
+        node_decremental = '<bs>',
+      },
+    },
+  },
+}