update docs for static LLVM linking

- Requirements: LLVM at /usr/local/llvm, Zig optional for cross-compilation
- Remove stale dylib references from all docs
- Fix CONTRIBUTING.md: correct repo URL, remove broken -o flag usage
- AGENTS.md: add LLVM linking section explaining static bridge

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Eric Hartford