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

lua/scl/builtin_instructions.lua

@@ -1,5 +1,9 @@
+-- TIA Portal built-in instructions for SCL Neovim plugin
+-- Defines functions, function blocks, and their parameters for completion
+
 local M = {}
 
+-- Built-in functions (return values, no state)
 M.FUNCTIONS = {
   ADD = true,
   SUB = true,
@@ -38,6 +42,7 @@ M.FUNCTIONS = {
   I_BCD = true,
 }
 
+-- Built-in function blocks (have state, need instance)
 M.FUNCTION_BLOCKS = {
   TON = true,
   TOF = true,
@@ -64,6 +69,8 @@ M.FUNCTION_BLOCKS = {
   DATALOG = true,
 }
 
+-- Parameter definitions for built-in instructions
+-- Format: { inputs = { { name, type }, ... }, outputs = { { name, type }, ... } }
 M.PARAMETERS = {
   TON = { inputs = { { name = "IN", type = "BOOL" }, { name = "PT", type = "TIME" } }, outputs = { { name = "Q", type = "BOOL" }, { name = "ET", type = "TIME" } } },
   TOF = { inputs = { { name = "IN", type = "BOOL" }, { name = "PT", type = "TIME" } }, outputs = { { name = "Q", type = "BOOL" }, { name = "ET", type = "TIME" } } },
@@ -121,14 +128,23 @@ M.PARAMETERS = {
   I_BCD = { inputs = { { name = "IN" } }, outputs = { { name = "OUT" } } },
 }
 
+--- Get parameter info for a built-in instruction
+--- @param name string The instruction name
+--- @return table|nil Parameter definition with inputs and outputs arrays
 function M.get_parameters(name)
   return M.PARAMETERS[name:upper()]
 end
 
+--- Check if name is a known function block
+--- @param name string The name to check
+--- @return boolean True if known function block
 function M.is_known_function_block(name)
   return M.FUNCTION_BLOCKS[name:upper()] or false
 end
 
+--- Check if name is a known function
+--- @param name string The name to check
+--- @return boolean True if known function
 function M.is_known_function(name)
   return M.FUNCTIONS[name:upper()] or false
 end