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/test_integration.lua

@@ -1,17 +1,11 @@
 local test = dofile("test/test_harness.lua")
 
-package.path = package.path .. ";lua/?.lua;src/?.lua"
+package.path = package.path .. ";lua/?.lua"
 
 local udt_parser = require("tia.udt_parser")
 local db_parser = require("tia.db_parser")
 local fb_parser = require("tia.fb_parser")
 
-local script_path = debug.getinfo(1, "S").source:gsub("^@", ""):match("(.*/)") or "."
-script_path = script_path:gsub("^test/", ""):gsub("/tests$", "")
-if script_path == "" then script_path = "." end
-
-local plc_json = dofile(script_path .. "/src/plc_json.lua")
-
 local function scan_directory(dir, ext)
   local files = {}
   local handle = io.popen('find "' .. dir .. '" -type f -name "*.' .. ext .. '" 2>/dev/null')
@@ -234,9 +228,6 @@ local function run_integration_tests()
 
     print("\n--- Type Resolution ---")
     test_type_resolution()
-
-    print("\n--- Workspace Type Loading ---")
-    test_workspace_type_loading()
   end)
 end