feat(poet): add statement types and pointer support, delete unused templates

Add AST support for structured function bodies:
- Return: return statement with multiple values
- For: traditional for loop and range-based iteration
- If: if/else statements with optional init clause

Enhance Param type:
- Add Pointer boolean field to indicate pointer types instead of
  requiring "*" prefix in the Type string

Delete unused templates:
- Remove internal/codegen/golang/templates/ directory containing
  .tmpl files that are no longer used after switching to poet-based
  code generation

Author: claude