## What changed
Move `HeadTailBuffer` and `format_output_omission_marker` into the new
`codex-utils-head-tail-buffer` crate, exposing the buffer API with an explicit
const-generic byte budget. Update core callers to use the crate and add Cargo
and Bazel definitions.
Move the existing buffer tests alongside the implementation, preserving the
prefix/suffix retention and omission-marker behavior.
GitOrigin-RevId: ff488d0a0a62c39c0e11c11ebb539166cc4c6e74
