chore: adjust README, AGENTS, and test runner for server-only scope

After splitting from the original scl_lsp repo, update documentation
and test infrastructure to reflect the server-only scope of this repo.

- README.md: focused on LSP server features, tree-sitter grammar,
  running the server, and Mason installation reference
- AGENTS.md: server-only project structure and conventions
- test/run_tests.lua: run only server tests (formatter, plc_json)

test/run_tests.lua

@@ -1,11 +1,11 @@
 #!/usr/bin/env lua
 
-package.path = package.path .. ";lua/?.lua;src/?.lua"
+package.path = package.path .. ";src/?.lua"
 
 local test = dofile("test/test_harness.lua")
 
 print("========================================")
-print("   SCL LSP - Test Suite")
+print("   tia-lsp - Server Test Suite")
 print("========================================")
 print(string.format("Reference Project: %s", test.REF_PROJECT))
 
@@ -25,12 +25,8 @@ local function run_all_tests()
     end
   end
 
-  run_test_file("test_udt.lua")
-  run_test_file("test_db.lua")
-  run_test_file("test_fb.lua")
   run_test_file("test_formatter.lua")
   run_test_file("test_plc_json.lua")
-  run_test_file("test_integration.lua")
 
   test.report()