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/custom/plugins/copilot.lua

@@ -1,35 +0,0 @@
-return {
-  -- Use copilot.lua for API access (no UI)
-  {
-    "zbirenbaum/copilot.lua",
-    cmd = "Copilot",
-    event = "InsertEnter",
-    config = function()
-      require("copilot").setup({
-        suggestion = { enabled = false }, -- Disable inline ghost text
-        panel = { enabled = false },      -- Disable suggestion panel
-        filetypes = {
-          yaml = false,
-          markdown = false,
-          help = false,
-          gitcommit = false,
-          gitrebase = false,
-          hgcommit = false,
-          svn = false,
-          cvs = false,
-          ["."] = false,
-        },
-        copilot_node_command = 'node', -- Node.js version must be > 18.x
-        server_opts_overrides = {},
-      })
-    end,
-  },
-  
-  -- Bridge between copilot.lua and blink.cmp
-  {
-    "giuxtaposition/blink-cmp-copilot",
-    dependencies = {
-      "zbirenbaum/copilot.lua",
-    },
-  },
-}