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/locals.scm

@@ -0,0 +1,15 @@
+; Local variable scoping for SCL - matches grammar.js structure
+
+; Variable declarations define local variables
+(var_item
+  name: (identifier) @definition.var)
+
+; Variable sections create scopes
+(var_declaration) @local.scope
+
+; Blocks create scopes
+(organization_block) @local.scope
+(function_block) @local.scope
+
+; Identifiers are references
+(identifier) @reference