Add GrammarIndex class that loads grammars.yml and provides fast
lookup by (package, context_symbol). Enables agents to get the right
GBNF grammar at code generation time.
API:
load_grammar_index(project_root) → GrammarIndex
idx.get(file_path, context_symbol=None) → gbnf_string
idx.get_package(file_path) → [(symbol, grammar, score, methods)]
14 tests, all passing.
