feat: add builtin types/functions to diagnostics and completions

Files Modified/Created:
- src/builtin_instructions.lua - NEW: Contains all TIA Portal built-in types, FBs, and functions
- src/diagnostics.lua - Added missing data types + builtin instruction checking
- src/main.lua - Added completion support for built-in types/F Bs/functions
- src/plc_json.lua - Added .db file parsing + BOM handling fixes
- lua/scl/blink_cmp_source.lua - Extended completion with all built-in types/F Bs/functions
- src/node-types.json - Added node types for parser
- AGENTS.md - Updated documentation
Features Added:
1. Diagnostics - No more false "Unknown data type" warnings for:
   - All elementary types (USINT, SINT, UINT, UDINT, LINT, ULINT, TOD, DTL, etc.)
   - Timer/counter types (IEC_TON, TOF, TP, CTU, CTD, CTUD)
   - TIA Portal built-in FBs and functions
2. Auto-completion - Shows in VAR section after ::
   - 28 elementary data types
   - 23 built-in function blocks (TON, TOF, TP, CTU, CTD, CTUD, R_TRIG, F_TRIG, etc.)
   - 80+ built-in functions (ADD, SUB, MUL, DIV, SIN, COS, SQRT, etc.)

src/main.lua

@@ -26,6 +26,7 @@ local treesitter = dofile(script_path .. "/treesitter.lua")
 local plc_json = dofile(script_path .. "/plc_json.lua")
 local diagnostics = dofile(script_path .. "/diagnostics.lua")
 local formatter = dofile(script_path .. "/formatter.lua")
+local builtin = dofile(script_path .. "/builtin_instructions.lua")
 
 local documents = {}
 
@@ -57,7 +58,7 @@ local semanticTokenTypes = {
 local capabilities = {
 	hoverProvider = true,
 	completionProvider = {
-		triggerCharacters = { ".", "#", "(" },
+		triggerCharacters = { ".", "#", "(", " ", ":", ";", "\n" },
 		resolveProvider = false,
 	},
 	definitionProvider = true,
@@ -385,6 +386,27 @@ function handlers.textDocument_completion(params)
 		local line = lines[line_idx]
 		local col = params.position.character
 		local trigger = col > 0 and line:sub(col, col) or ""
+		local line_before = col > 1 and line:sub(1, col - 1) or ""
+
+		-- Get word prefix being typed (for filtering)
+		local prefix_start = col - 1
+		while prefix_start > 0 and line:sub(prefix_start, prefix_start):match("[%w_]") do
+			prefix_start = prefix_start - 1
+		end
+		local word_prefix = line:sub(prefix_start + 1, col - 1)
+
+		-- Check if we're in VAR section
+		local in_var_section = false
+		for i = line_idx - 1, 1, -1 do
+			local prev_line = lines[i]:gsub("^%s+", ""):gsub("%s+$", "")
+			if prev_line:match("^END_VAR") or prev_line:match("^VAR_TEMP") or prev_line:match("^VAR_IN_OUT") or prev_line:match("^VAR_OUTPUT") or prev_line:match("^VAR_INPUT") then
+				break
+			end
+			if prev_line:match("^VAR%s*$") or prev_line:match("^VAR$") then
+				in_var_section = true
+				break
+			end
+		end
 
 		if trigger == "#" then
 			if doc.variables then
@@ -450,6 +472,53 @@ function handlers.textDocument_completion(params)
 					})
 				end
 			end
+		else
+			-- Check context before cursor
+			local is_var_decl = line_before:match(":%s*$")
+
+			-- Helper to add item with prefix filtering
+			local function add_item(name, kind_val, detail_text)
+				if word_prefix == "" or name:upper():find("^" .. word_prefix:upper()) then
+					table.insert(items, {
+						label = name,
+						kind = kind_val,
+						detail = detail_text,
+						insertText = name,
+						insertTextFormat = 1,
+					})
+				end
+			end
+
+			if in_var_section or is_var_decl then
+				-- In VAR section: show data types and function blocks
+				for type_name, _ in pairs(builtin.DATA_TYPES) do
+					add_item(type_name, 7, "Data type")
+				end
+
+				for fb_name, _ in pairs(builtin.FUNCTION_BLOCKS) do
+					add_item(fb_name, 6, "Function block")
+				end
+
+				-- Also add workspace types if available
+				if doc.types then
+					for type_name, type_info in pairs(doc.types) do
+						add_item(type_name, 7, type_info.kind or "Type")
+					end
+				end
+			else
+				-- General context: show all
+				for type_name, _ in pairs(builtin.DATA_TYPES) do
+					add_item(type_name, 7, "Data type")
+				end
+
+				for fb_name, _ in pairs(builtin.FUNCTION_BLOCKS) do
+					add_item(fb_name, 6, "Function block")
+				end
+
+				for func_name, _ in pairs(builtin.FUNCTIONS) do
+					add_item(func_name, 3, "Function")
+				end
+			end
 		end
 	end