feat: add Kotlin language support

Add Kotlin (.kt, .kts) as the 13th supported language with full
tree-sitter parsing, AST node extraction, call resolution, usage
tracking, and string constant propagation.
Changes:
- internal/lang/kotlin.go: Kotlin LanguageSpec registration
- internal/lang/lang.go: Add Kotlin constant and AllLanguages()
- internal/parser/parser.go: Register tree-sitter-kotlin grammar
- internal/pipeline/pipeline.go: Kotlin in isConstantNode(),
  isExported(), extractCalleeName()
- internal/pipeline/resolve.go: resolveKotlin() for val declarations
- internal/pipeline/usages.go: Kotlin reference node types
- Tests: TestParseKotlin, TestResolveKotlinConcat, extension tests
- go.mod: Add tree-sitter-grammars/tree-sitter-kotlin v1.1.0
- README.md: Update language count to 13

Author: GauravRatnawat

internal/lang/lang.go

@@ -88,4 +88,4 @@ func LanguageForExtension(ext string) (Language, bool) {
 		return "", false
 	}
 	return spec.Language, true
-}
+}

internal/parser/parser_test.go

@@ -200,4 +200,4 @@ func Hello() string {
 		}
 		return true
 	})
-}
+}

internal/pipeline/resolve.go

@@ -801,4 +801,4 @@ func firstNonTrivialChild(node *tree_sitter.Node) *tree_sitter.Node {
 		}
 	}
 	return nil
-}
+}