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/indent-line.lua

@@ -0,0 +1,33 @@
+-- Indent guides - Show vertical lines at indentation levels
+return {
+  'lukas-reineke/indent-blankline.nvim',
+  event = { 'BufReadPost', 'BufNewFile' },
+  main = 'ibl',
+  opts = {
+    indent = {
+      char = '│',
+      tab_char = '│',
+    },
+    scope = {
+      enabled = true,
+      show_start = true,
+      show_end = false,
+      injected_languages = false,
+      highlight = { 'Function', 'Label' },
+    },
+    exclude = {
+      filetypes = {
+        'help',
+        'alpha',
+        'dashboard',
+        'neo-tree',
+        'Trouble',
+        'lazy',
+        'mason',
+        'notify',
+        'toggleterm',
+        'lazyterm',
+      },
+    },
+  },
+}