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

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

queries/scl/indents.scm

@@ -0,0 +1,41 @@
+; Indentation rules for SCL
+
+; Indent after block keywords
+[
+  (organization_block)
+  (function_block)
+  (var_declaration)
+  (if_statement)
+  (case_statement)
+  (for_statement)
+  (while_statement)
+  (repeat_statement)
+  (region_statement)
+] @indent.begin
+
+; Indent after THEN, DO, etc.
+[
+  "THEN"
+  "DO"
+  "ELSE"
+  "ELSIF"
+] @indent.branch
+
+; Outdent at end keywords
+[
+  "END_VAR"
+  "END_IF"
+  "END_CASE"
+  "END_FOR"
+  "END_WHILE"
+  "END_REPEAT"
+  "END_REGION"
+  "END_ORGANIZATION_BLOCK"
+  "END_FUNCTION_BLOCK"
+] @indent.end
+
+; Outdent for ELSE and ELSIF
+[
+  "ELSE"
+  "ELSIF"
+] @indent.dedent

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

queries/scl/tags.scm

@@ -0,0 +1,27 @@
+; Tag queries for SCL - matches grammar.js structure
+
+; Organization blocks
+(organization_block
+  name: (string) @name) @definition.function
+
+; Function blocks  
+(function_block
+  name: (string) @name) @definition.function
+
+; Variable definitions within blocks
+(var_item
+  name: (identifier) @name
+  type: (type) @type) @definition.variable
+
+; Parameter definitions (inputs/outputs)
+(var_declaration
+  (var_item
+    name: (identifier) @name
+    type: (type) @type)) @definition.variable
+
+; Function calls (references)
+(function_call
+  (identifier) @name) @reference.call
+
+(function_call
+  (string) @name) @reference.call