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/core/bootstrap.lua

@@ -0,0 +1,13 @@
+-- Lazy.nvim installation
+local lazypath = vim.fn.stdpath 'data' .. '/lazy/lazy.nvim'
+if not vim.loop.fs_stat(lazypath) then
+  vim.fn.system {
+    'git',
+    'clone',
+    '--filter=blob:none',
+    'https://github.com/folke/lazy.nvim.git',
+    '--branch=stable',
+    lazypath,
+  }
+end
+vim.opt.rtp:prepend(lazypath)

lua/core/health.lua

@@ -0,0 +1,98 @@
+-- Health check for the modular Neovim configuration
+
+local M = {}
+
+local check_version = function()
+  local verstr = tostring(vim.version())
+  if not vim.version.ge then
+    vim.health.error(string.format("Neovim out of date: '%s'. Upgrade to latest stable or nightly", verstr))
+    return
+  end
+
+  if vim.version.ge(vim.version(), '0.10-dev') then
+    vim.health.ok(string.format("Neovim version is: '%s'", verstr))
+  else
+    vim.health.error(string.format("Neovim out of date: '%s'. Upgrade to latest stable or nightly", verstr))
+  end
+end
+
+local check_external_reqs = function()
+  -- Basic utils
+  for _, exe in ipairs { 'git', 'make', 'unzip' } do
+    local is_executable = vim.fn.executable(exe) == 1
+    if is_executable then
+      vim.health.ok(string.format("Found executable: '%s'", exe))
+    else
+      vim.health.warn(string.format("Could not find executable: '%s'", exe))
+    end
+  end
+  
+  -- Search tools
+  for _, exe in ipairs { 'rg', 'fd' } do
+    local is_executable = vim.fn.executable(exe) == 1
+    if is_executable then
+      vim.health.ok(string.format("Found search tool: '%s'", exe))
+    else
+      vim.health.warn(string.format("Could not find search tool: '%s' (required for Telescope)", exe))
+    end
+  end
+end
+
+local check_lsp_servers = function()
+  -- Check for LSP servers installed via Nix
+  local servers = {
+    { name = 'clangd', desc = 'C/C++ language server' },
+    { name = 'pyright', desc = 'Python language server' },
+    { name = 'ruff', desc = 'Python linter/formatter' },
+    { name = 'nixd', desc = 'Nix language server' },
+    { name = 'texlab', desc = 'LaTeX language server' },
+    { name = 'cmake-language-server', desc = 'CMake language server' },
+  }
+  
+  vim.health.start('LSP Servers (via Nix/Home Manager)')
+  for _, server in ipairs(servers) do
+    local is_executable = vim.fn.executable(server.name) == 1
+    if is_executable then
+      vim.health.ok(string.format("Found %s: '%s'", server.desc, server.name))
+    else
+      vim.health.info(string.format("Not found: '%s' (%s) - install via Nix if needed", server.name, server.desc))
+    end
+  end
+end
+
+local check_formatters = function()
+  -- Check for formatters installed via Nix
+  local formatters = {
+    { name = 'stylua', filetype = 'lua' },
+    { name = 'clang-format', filetype = 'c/cpp' },
+    { name = 'alejandra', filetype = 'nix' },
+  }
+  
+  vim.health.start('Formatters (via Nix/Home Manager)')
+  for _, formatter in ipairs(formatters) do
+    local is_executable = vim.fn.executable(formatter.name) == 1
+    if is_executable then
+      vim.health.ok(string.format("Found formatter for %s: '%s'", formatter.filetype, formatter.name))
+    else
+      vim.health.info(string.format("Not found: '%s' (%s) - install via Nix if needed", formatter.name, formatter.filetype))
+    end
+  end
+end
+
+function M.check()
+  vim.health.start('Modular Neovim Configuration')
+
+  vim.health.info [[NOTE: Not every warning needs to be fixed.
+Only install tools for languages you actually use.
+All language servers and formatters should be installed via Nix/Home Manager.]]
+
+  local uv = vim.uv or vim.loop
+  vim.health.info('System Information: ' .. vim.inspect(uv.os_uname()))
+
+  check_version()
+  check_external_reqs()
+  check_lsp_servers()
+  check_formatters()
+end
+
+return M

lua/core/keymaps.lua

@@ -0,0 +1,26 @@
+-- [[ Basic Keymaps ]]
+-- See `:help vim.keymap.set()`
+
+-- LSP reload function
+local function reload_lsp()
+  local clients = vim.lsp.get_clients()
+  if #clients == 0 then
+    print('No LSP clients running')
+    return
+  end
+  
+  for _, client in ipairs(clients) do
+    vim.lsp.stop_client(client.id)
+  end
+  
+  vim.defer_fn(function()
+    vim.cmd('LspStart')
+    print('LSP servers reloaded')
+  end, 500)
+end
+
+-- Reload LSP keybind
+vim.keymap.set('n', '<leader>lr', reload_lsp, { desc = '[L]SP [R]eload all servers' })
+
+-- Standard practice for Lua modules that don't need to return complex data
+return {}

lua/core/options.lua

@@ -0,0 +1,69 @@
+-- Place custom vim options here
+
+-- Set based on your font installation
+vim.g.have_nerd_font = true
+
+-- Indentation settings
+vim.o.smartindent = true
+vim.o.autoindent = true
+vim.o.expandtab = true
+vim.o.tabstop = 2
+vim.o.shiftwidth = 2
+vim.o.softtabstop = 2
+
+-- Add any other custom vim.o or vim.g settings from your old config here
+-- For example, if you changed defaults for:
+-- vim.opt.number = true -- (Already default in kickstart)
+-- vim.opt.mouse = 'a' -- (Already default in kickstart)
+-- etc... Review the options section of your old init.lua and add any *changed* values here.
+-- The kickstart defaults are generally sensible, so you might not need many overrides.
+
+-- Function to check if running in a shared tmux session
+local function is_shared_tmux_session()
+  if not vim.env.TMUX then
+    return false
+  end
+  
+  local handle = io.popen('tmux list-sessions -F "#{session_name}:#{session_attached}" 2>/dev/null')
+  if not handle then
+    return false
+  end
+  
+  local current_session = vim.fn.system('tmux display-message -p "#S"'):gsub('\n', '')
+  local output = handle:read('*a')
+  handle:close()
+  
+  -- Check if session name contains "shared" (case insensitive)
+  if current_session:lower():find('shared') then
+    return true
+  end
+  
+  -- Also check if multiple users are attached
+  for line in output:gmatch('[^\r\n]+') do
+    local session_name, attached_count = line:match('([^:]+):(%d+)')
+    if session_name == current_session and tonumber(attached_count) > 1 then
+      return true
+    end
+  end
+  
+  return false
+end
+
+-- Warn before quitting if in a shared tmux session
+vim.api.nvim_create_autocmd('VimLeavePre', {
+  callback = function()
+    if is_shared_tmux_session() then
+      local choice = vim.fn.confirm(
+        'You are in a shared tmux session. Other users may be affected.\nDo you really want to quit?',
+        '&Yes\n&No',
+        2  -- Default to No
+      )
+      -- vim.fn.confirm returns 1 for Yes, 2 for No, 0 for Esc
+      -- The & makes Y/y and N/n work as shortcuts (case-insensitive)
+      if choice ~= 1 then
+        return  -- Prevent quit unless explicitly choosing Yes
+      end
+    end
+  end,
+})
+