perf: async cache initialization and remove deepcopy (#8)

* perf: async cache initialization and remove deepcopy

Problem: first completion request blocked the UI with a synchronous
vim.system():wait() call, and every subsequent key completion
unnecessarily deep-copied the entire cache.

Solution: use vim.system with an async callback to initialize the cache
without blocking. Queue pending completion requests during loading and
serve them once parsing finishes. Return cached keys directly instead
of deep-copying.

* fix: revert blanket diagnostics.disable and selene comments

Problem: .luarc.json blanket-disabled four diagnostic categories
project-wide, and selene inline directives were added to suppress
warnings on io.open monkey-patching in tests.

Solution: revert .luarc.json to match main and remove selene comments.

spec/ghostty_spec.lua

@@ -19,24 +19,39 @@ local BASH_COMPLETION = table.concat({
 }, '\n')
 
 local function mock_system()
-  local original = vim.system
+  local original_system = vim.system
+  local original_schedule = vim.schedule
   ---@diagnostic disable-next-line: duplicate-set-field
-  vim.system = function(cmd)
+  vim.system = function(cmd, _, on_exit)
     if cmd[1] == 'ghostty' then
+      local result = { stdout = CONFIG_DOCS, code = 0 }
+      if on_exit then
+        on_exit(result)
+        return {}
+      end
       return {
         wait = function()
-          return { stdout = CONFIG_DOCS, code = 0 }
+          return result
         end,
       }
     end
+    local result = { stdout = '', code = 1 }
+    if on_exit then
+      on_exit(result)
+      return {}
+    end
     return {
       wait = function()
-        return { stdout = '', code = 1 }
+        return result
       end,
     }
   end
+  vim.schedule = function(fn)
+    fn()
+  end
   return function()
-    vim.system = original
+    vim.system = original_system
+    vim.schedule = original_schedule
   end
 end