Follow-up to the cherry-picked fix from #112724 (@poijygfdyy):
- tools/checkpoint_profile_migration.py -> tools/checkpoint_manager_profile_rename.py, the
repo's `<stem>_<topic>.py` sibling convention for code that extends checkpoint_manager.
- Replace the fail-closed target-collision check plus temp-file/rollback choreography with an
idempotent rekey: every step overwrites and the old project metadata is removed last, so a
mid-way failure is repaired by `hermes profile migrate-identity` redoing the same writes.
A genuine collision cannot occur — `profiles/<new>` must not exist for the rename to run.
192 -> 98 lines.
- Metadata/ledger writes go through the same idiom as checkpoint_manager itself
(`_register_project` plain write, `_save_ledger`), dropping the private temp-file helpers.
- Keep the git-present precondition as a single early check: without git the ref cannot move
and rekeying only the metadata would orphan the history.
- Test: `create_profile` now seeds `workspace/`, so the fixture uses `project/`; add the control
assertion that a workdir outside the profile dir keeps its history unchanged.
- Docs: the profile rename / migrate-identity reference notes that checkpoint history is preserved.
Fixes#112973
c055eee1fe