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/git.lua

@@ -0,0 +1,26 @@
+-- Git integration plugins
+return {
+  -- Fugitive - Git integration
+  {
+    'tpope/vim-fugitive',
+    cmd = { 'Git', 'G', 'Gdiff', 'Gread', 'Gwrite', 'Ggrep', 'GMove', 'GDelete', 'GBrowse', 'GRemove' },
+    keys = {
+      { '<leader>gs', '<cmd>Git<cr>', desc = 'Git status' },
+      { '<leader>gd', '<cmd>Gdiff<cr>', desc = 'Git diff' },
+      { '<leader>gc', '<cmd>Git commit<cr>', desc = 'Git commit' },
+      { '<leader>gb', '<cmd>Git blame<cr>', desc = 'Git blame' },
+      { '<leader>gl', '<cmd>Git log<cr>', desc = 'Git log' },
+      { '<leader>gp', '<cmd>Git push<cr>', desc = 'Git push' },
+      { '<leader>gf', '<cmd>Git fetch<cr>', desc = 'Git fetch' },
+    },
+  },
+  
+  -- Gitsigns - Git gutter and hunk operations
+  {
+    'lewis6991/gitsigns.nvim',
+    event = 'VeryLazy',
+    config = function()
+      require('plugins.config.git').setup_gitsigns()
+    end,
+  },
+}