Add a README, and move agent guidance into AGENTS.md

[bfierz]

Summary

• README.md (new): what FloatWeaver is, with two short examples (float8 arithmetic and an Eigen cross product), features, supported WEAVER_VECTORIZE backends, building with or without Conan, running the tests, building the docs, and acknowledgements for the SIMD math functions.
  • Origin section: FloatWeaver began as the SIMD module of VCL and is a from-scratch C++17 reimplementation of it as a standalone library.
• AGENTS.md (new): project guidance for coding agents (background, build and test commands, architecture), written so any agent can use it. References to the vcl source tree are removed.
• CLAUDE.md (new): only imports AGENTS.md, so Claude Code gets the same guidance.

Notes for review

• The README doesn't link to the hosted docs yet, because the GitHub Pages URL isn't confirmed. Worth adding once it is.
• The build commands, backend list and CI coverage were taken from CMakeLists.txt, cmake/config.cmake and .github/workflows/. The code examples come from doc/snippets.