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/diagnostics.lua

@@ -16,6 +16,7 @@ end
 package.path = package.path .. ";" .. script_path .. "/?.lua"
 
 local parser = dofile(script_path .. "/parser.lua")
+local builtin = dofile(script_path .. "/builtin_instructions.lua")
 
 local DiagnosticSeverity = {
   Error = 1,
@@ -156,9 +157,16 @@ function M.validate_diagnostics(content, workspace_types, root_dir)
              upper_type ~= "WSTRING" and upper_type ~= "TIME" and upper_type ~= "DATE" and
              upper_type ~= "TIME_OF_DAY" and upper_type ~= "DATE_AND_TIME" and
              upper_type ~= "S5TIME" and upper_type ~= "LTIME" and
+             upper_type ~= "USINT" and upper_type ~= "SINT" and upper_type ~= "UINT" and
+             upper_type ~= "UDINT" and upper_type ~= "LINT" and upper_type ~= "ULINT" and
+             upper_type ~= "TOD" and upper_type ~= "DTL" and
              upper_type ~= "IEC_TIMER" and upper_type ~= "TON_TIME" and
              upper_type ~= "TOF_TIME" and upper_type ~= "TONR_TIME" and
-             upper_type ~= "TP_TIME" then
+             upper_type ~= "TP_TIME" and upper_type ~= "IEC_TON" and
+             upper_type ~= "IEC_TOF" and upper_type ~= "IEC_TP" and
+             upper_type ~= "IEC_COUNTER" and upper_type ~= "CTU" and
+             upper_type ~= "CTD" and upper_type ~= "CTUD" and
+             not builtin.is_known_type_or_instruction(upper_type) then
             table.insert(diagnostics, {
               range = range_to_lsp(line_num, 0, line_num, #line),
               severity = DiagnosticSeverity.Warning,