feat: add Clojure language support (tree-sitter + clojure-lsp)

[jafreck]

Summary

Add Clojure support via tree-sitter + LSP enrichment, complementing existing Java/Kotlin/Scala support in the JVM ecosystem.

Tooling

Component	Tool	Notes
Tree-sitter grammar	tree-sitter-clojure	Stable, well-tested
LSP server	clojure-lsp	Excellent, 1.5k+ stars, very active
SCIP indexer	None	N/A

What to implement

• Add tree-sitter-clojure as an npm dependency
• Create src/indexer/extractors/clojure.ts:
  • Extract (defn name ...) as function symbols
  • Extract (def name ...) as variable/constant symbols
  • Extract (defmacro name ...) as macro symbols
  • Extract (defprotocol Name ...) as interface symbols
  • Extract (defrecord Name ...) and (deftype Name ...) as class/type symbols
  • Extract (defmethod name dispatch-val ...) as method symbols
  • Extract function invocations (first element of list forms) as call refs
  • Extract (require '[ns.path :as alias]), (use ...), (import ...) as imports
  • Extract (ns name (:require ...)) namespace declaration
• Register clojure in SourceIndexStage EXTRACTORS map and ParserPool LANG_PACKAGES
• Add clojure-lsp to src/indexer/lsp/registry.ts: { command: 'clojure-lsp', args: [] }
• Add file extensions .clj, .cljs, .cljc, .edn to walker
• Add test fixtures in tests/fixtures/clojure/
• Add extractor tests

Language notes

• Clojure is homoiconic — code is data. Function calls are (function-name arg1 arg2) (first element of a list)
• Namespace-qualified symbols: my.namespace/my-fn — the extractor should preserve the full qualified name and also record the bare name
• .clj = JVM Clojure, .cljs = ClojureScript (JavaScript target), .cljc = reader conditionals (both targets)
• Thread-first -> and thread-last ->> macros create call chains analogous to pipe operators
• Multimethod dispatch (defmulti/defmethod) creates virtual-dispatch relationship edges
• clojure-lsp provides excellent go-to-definition, find-references, and rename across namespaces

Context

Complements existing Java/Kotlin/Scala support. Clojure runs on the JVM and frequently interops with Java libraries. ClojureScript shares dependencies with JavaScript/TypeScript projects.