chore: adjust for plugin-only scope after split

After splitting from the original scl_lsp repo, update for plugin-only scope:

- README.md: focused on Neovim plugin installation, configuration,
  commands, keybindings, and editor features
- AGENTS.md: plugin-only project structure and conventions
- test/run_tests.lua: run only plugin tests (udt, db, fb, integration)
- test/test_integration.lua: remove plc_json dependency (server module)
- queries/: move from queries/*.scm to queries/scl/*.scm (nvim-treesitter
  per-language convention)
- ftdetect/scl.vim: add filetype detection for .scl/.udt/.db files

test/run_tests.lua

@@ -1,11 +1,11 @@
 #!/usr/bin/env lua
 
-package.path = package.path .. ";lua/?.lua;src/?.lua"
+package.path = package.path .. ";lua/?.lua"
 
 local test = dofile("test/test_harness.lua")
 
 print("========================================")
-print("   SCL LSP - Test Suite")
+print("   tia-lsp.nvim - Plugin Test Suite")
 print("========================================")
 print(string.format("Reference Project: %s", test.REF_PROJECT))
 
@@ -28,8 +28,6 @@ local function run_all_tests()
   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()