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

queries/scl/folds.scm

@@ -0,0 +1,14 @@
+; Folds for SCL code blocks
+
+[
+  (organization_block)
+  (function_block)
+  (var_declaration)
+  (if_statement)
+  (case_statement)
+  (for_statement)
+  (while_statement)
+  (repeat_statement)
+  (region_statement)
+  (block_comment)
+] @fold