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/formatting.lua

@@ -0,0 +1,40 @@
+-- Formatter configuration
+
+return {
+    -- ========================================
+    -- Formatter Configuration (conform.nvim)
+    -- ========================================
+    {
+        'stevearc/conform.nvim',
+        event = 'BufWritePre', -- Format on save
+        -- cmd = { 'ConformInfo' }, -- Optional: If you want the command available
+        -- keys = { ... } -- Optional: Define keys if needed
+        opts = {
+            formatters_by_ft = {
+                lua = { 'stylua' },
+                c = { 'clang_format' },
+                cpp = { 'clang_format' },
+                -- Use ruff for Python formatting (includes isort and is faster than black
+                -- Ensure 'ruff' is installed via Home Manager (pkgs.ruff)
+                python = { 'ruff_format', 'ruff_fix' },
+                -- python = { 'isort', 'black' },
+                nix = { 'alejandra' }, -- Add nix formatter
+                -- Add other filetypes and formatters, e.g.:
+                -- javascript = { "prettier" },
+                -- typescript = { "prettier" },
+                -- css = { "prettier" },
+                -- html = { "prettier" },
+                -- json = { "prettier" },
+                -- yaml = { "prettier" },
+                -- markdown = { "prettier" },
+                -- bash = { "shfmt" },
+            },
+            -- Configure format_on_save behavior
+            format_on_save = {
+                -- I recommend these options, but adjust to your liking
+                timeout_ms = 500,    -- Stop formatting if it takes too long
+                lsp_fallback = true, -- Fallback to LSP formatting if conform fails
+            },
+        },
+    },
+}