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/highlights.scm

@@ -0,0 +1,95 @@
+; SCL highlights
+
+(line_comment) @comment
+(block_comment) @comment
+(c_style_comment) @comment
+
+(number) @number
+(hex_number) @number
+(string) @string
+(time_value) @number
+
+(prefix) @punctuation.special
+
+(identifier) @variable
+
+; FB instance calls - highlight instance name as type
+(fb_call
+  instance: (prefixed_identifier
+    (identifier) @type))
+(fb_call
+  instance: (identifier) @type)
+
+; FB parameter names (IN :=, PT :=, etc.)
+(fb_parameter
+  name: (identifier) @property)
+
+; Function call names
+(function_call
+  (identifier) @function)
+
+; Variable declarations
+(var_item
+  name: (identifier) @variable)
+
+; Type references in declarations
+(type
+  (identifier) @type)
+
+(type_builtin) @type.builtin
+
+(keyword) @keyword
+(constant) @constant.builtin
+(operator) @operator
+
+(assignment) @operator
+(binary_expression) @operator
+(unary_expression) @operator
+
+(field_access
+  "."
+  (identifier) @property)
+
+(array_access) @property
+
+(attribute_list) @attribute
+(attribute) @attribute
+(attribute_value) @string
+
+(var_item) @property
+
+(function_call) @function.call
+(fb_call) @function.call
+
+(if_statement) @conditional
+(elsif_clause) @conditional
+(else_clause) @conditional
+(case_statement) @conditional
+(case_item) @conditional
+
+(for_statement) @repeat
+(while_statement) @repeat
+(repeat_statement) @repeat
+
+";" @punctuation.delimiter
+":" @punctuation.delimiter
+"," @punctuation.delimiter
+"(" @punctuation.bracket
+")" @punctuation.bracket
+"[" @punctuation.bracket
+"]" @punctuation.bracket
+"." @punctuation.delimiter
+
+":=" @operator
+"=>" @operator
+"=" @operator
+"<>" @operator
+"<" @operator
+">" @operator
+"<=" @operator
+">=" @operator
+
+"+" @operator
+"-" @operator
+"*" @operator
+"/" @operator