style: use east const in the C API header

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

AGENTS.md

@@ -133,8 +133,8 @@ the custom scripts instead of system-installed packages.
 
 - Pure C only: no C++ syntax. The header is imported into Swift via
   `-import-objc-header`, so it must compile as C.
-- Standard C declaration style; trailing returns and east const do not
-  apply here.
+- Standard C declaration style with east const (e.g. `char const*`);
+  trailing returns do not apply here.
 - Use `stdbool.h` types so Swift sees `Bool`.
 
 ## Shell Scripts