feat(health): add checkhealth integration

Problem: need a way to diagnose config, data file, and
dependency issues.

Solution: add health module reporting config values, data
directory status, task count, and curl/openssl availability.

lua/todo/health.lua

@@ -0,0 +1,55 @@
+local M = {}
+
+function M.check()
+  vim.health.start('todo.nvim')
+
+  local ok, config = pcall(require, 'todo.config')
+  if not ok then
+    vim.health.error('Failed to load todo.config')
+    return
+  end
+
+  local cfg = config.get()
+  vim.health.ok('Config loaded')
+  vim.health.info('Data path: ' .. cfg.data_path)
+  vim.health.info('Default view: ' .. cfg.default_view)
+  vim.health.info('Default category: ' .. cfg.default_category)
+  vim.health.info('Date format: ' .. cfg.date_format)
+  vim.health.info('Date syntax: ' .. cfg.date_syntax)
+
+  local data_dir = vim.fn.fnamemodify(cfg.data_path, ':h')
+  if vim.fn.isdirectory(data_dir) == 1 then
+    vim.health.ok('Data directory exists: ' .. data_dir)
+  else
+    vim.health.warn('Data directory does not exist yet: ' .. data_dir)
+  end
+
+  if vim.fn.filereadable(cfg.data_path) == 1 then
+    local store_ok, store = pcall(require, 'todo.store')
+    if store_ok then
+      local load_ok, err = pcall(store.load)
+      if load_ok then
+        local tasks = store.tasks()
+        vim.health.ok('Data file loaded: ' .. #tasks .. ' tasks')
+      else
+        vim.health.error('Failed to load data file: ' .. tostring(err))
+      end
+    end
+  else
+    vim.health.info('No data file yet (will be created on first save)')
+  end
+
+  if vim.fn.executable('curl') == 1 then
+    vim.health.ok('curl found (required for Google Calendar sync)')
+  else
+    vim.health.warn('curl not found (needed for Google Calendar sync)')
+  end
+
+  if vim.fn.executable('openssl') == 1 then
+    vim.health.ok('openssl found (required for OAuth PKCE)')
+  else
+    vim.health.warn('openssl not found (needed for Google Calendar OAuth)')
+  end
+end
+
+return M