Move codeflash's own benchmarks to .codeflash/benchmarks/. Add
auto-discovery of .codeflash/benchmarks/ in codeflash compare and
benchmark mode -- when benchmarks-root is not explicitly configured,
the CLI checks for .codeflash/benchmarks/ before erroring.
Backwards compatible: users with existing benchmarks-root config
are unaffected. Docs continue to show tests/benchmarks as the
example path.