This adds llama.cpp support for classifiers. Classifier models (Julia-1, Laya, Kev, lev, OpenJev) are listed as typesafe-system-one classifiers instead of chat models; other models keep the llama-cpp-classify fallback. Only 501 and 404 mark a model as chat, and classifier models restored from the model store are checked again once loaded.
Built-in extensions (mcp, llama.cpp, codemode, tool-search) are now extension resources like files: the package manager resolves them as builtin:<name>, the extensions setting disables them with -builtin:<name> (per project too), pi config lists them, and --no-extensions turns them off unless loaded with -e builtin:<name>. They load after project trust is resolved and after file and package extensions.
Built-in extensions and tools are named builtin:<name> in errors, diagnostics, RPC source info, and bug reports.
Add the llama-cpp-classify classifier API. Each question becomes one chat prompt with single-token answer labels; llama-server's pre-sampling next-token log-probabilities give the softmax over those labels. Missing labels are retried with deeper readouts and then reported as errors.
