[codex] add VFS agent guidance files

[GavinPacini]

Summary

• Add a single embedded markdown source for agent guidance in the VFS docs tree.
• Expose identical read-only root files at /AGENTS.md and /CLAUDE.md.
• Cover root listing and beth vfs cat access in router and CLI tests.

Impact

Agents opening a mounted beth VFS can immediately find guidance at the root, and the same content is available without a mount through beth vfs cat /AGENTS.md or /CLAUDE.md. The guidance explicitly points readers to /docs for more information.

Validation

• cargo test -p beth-vfs
• cargo test -p beth

Closes #4.