feat: add Clojure/Nim chapter to the book + restructure for GitLab

- New chapter 05-clojure-nim.md (EN + BG) covering:
  - Native compilation pipeline (Clojure → Nim → C → binary)
  - AI-powered development (error explanation, code generation)
  - JSON REPL for AI agents
  - loop/recur with real TCO
  - Cross-compilation: JS, shared libs, WASM
  - Persistent data structures (HAMT)
  - Concurrency: atoms, agents, channels
- Updated book README.md with Clojure/Nim focus
- Added Clojure/Nim terms to subject indices (EN + BG)
- Removed books/ from .gitignore so it can be pushed to GitLab

.gitignore

@@ -14,4 +14,4 @@ test_deps
 test_eval
 test_macros
 test_ai_assist
-books/
+