Convert the remaining west-const declarations in xone_api.h to east
const so the whole file matches the controller functions added later.
Update the AGENTS.md C bridge section: east const applies to the C
header too; only trailing returns are exempt.
Co-Authored-By: qwen3.8-27b@q3_k_xl: converted C API header to east const
Adapt the refix AGENTS.md to this project: build commands, module
layout, C++/Swift/C-bridge style rules, and commit guidelines. C++
headers use .hpp and C headers use .h; the PLAN.md port table is
updated to match.
Co-Authored-By: qwen (qwen/qwen3.8-27b@q2_k_xl): wrote coding conventions
portersky