feat: add :CoderTransform command for inline tag replacement

New feature: Transform /@ @/ tags directly in any file, not just
.coder.* files. The tags are replaced inline with LLM-generated code.

New commands:
- :CoderTransform - Transform ALL /@ @/ tags in current file
- :CoderTransformCursor - Transform only the tag at cursor position
- :Coder transform - Same as :CoderTransform
- :Coder transform-cursor - Same as :CoderTransformCursor

How it works:
1. Write /@ your prompt @/ anywhere in your code
2. Run :CoderTransform
3. The tag is replaced with generated code matching your file's style

Example:
  /@ create a function to validate email @/

  Becomes:
  function validateEmail(email) {
    return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
  }

lua/codetyper/prompts/code.lua

@@ -4,6 +4,13 @@
 
 local M = {}
 
+/@ 
+create a prompt to generate code based on user description
+-- @usage M.create_function
+-- @param description string Description of the code to generate
+-- @return string Prompt template
+@/
+
 --- Prompt template for creating a new function
 M.create_function = [[Create a function with the following requirements: