libgit2 creates loose objects and packfiles with mode 0444 (read-only). FATFS
turns that into the AM_RDO attribute and then refuses to f_unlink them (EACCES),
and esp-idf's FATFS VFS chmod cannot clear AM_RDO — so a written object can
never be deleted, blocking re-clone recovery and (later) fetch/repack.
Shim p_open/p_creat in esp_stubs.c to force owner-write into every create mode
(compile posix.c's originals under throwaway names, same mechanism as p_rename),
so nothing is ever created read-only. Immutability is only a safety hint on an
appliance where nothing but libgit2 touches these files.
Non-regressive on hardware: git_push, which links the same component, still
commits + pushes cleanly with the shim. The delete path itself is validated via
git_sync's RECLONE_EACH_BOOT knob (pending a dedicated flash).
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