docs: encourage adding comments; fix nested IF/ELSIF indentation

- Update AGENTS.md to encourage comments instead of forbidding them
- Add docstrings to all public functions in src/ modules
- Add module headers and function comments to lua/scl/ modules
- Fix formatter to properly indent multi-line IF/ELSIF conditions
- Fix FB call detection to not match IF statements with parentheses

src/parser.lua

@@ -1,6 +1,12 @@
 -- SCL Parser utilities for the LSP server
+-- Extracts variables, types, and function definitions from SCL source code
+
 local M = {}
 
+--- Extract variables from SCL content
+--- @param content string The SCL source code
+--- @return table variables Table of variable definitions keyed by name
+--- @return table variable_positions Table of variable positions keyed by name
 function M.extract_variables(content)
   local variables = {}
   local variable_positions = {}
@@ -164,6 +170,9 @@ function M.extract_variables(content)
   return variables, variable_positions
 end
 
+--- Extract type definitions from SCL content
+--- @param content string The SCL source code
+--- @return table types Table of type definitions keyed by name
 function M.extract_types(content)
   local types = {}
 
@@ -285,6 +294,9 @@ function M.extract_types(content)
   return types
 end
 
+--- Extract function/block definitions from SCL content
+--- @param content string The SCL source code
+--- @return table functions Array of function definitions with name, kind, start, final
 function M.extract_functions(content)
   local functions = {}