bootstrap.py: _kept_name skips a -kept name with a branch stored under it, and a test row for a twin with a slash (follow-up of #962) #1000

Follow-up of #962 (landed as #984, 2a78ae1a): the two NITs of rv984b's READY re-check (#984 issuecomment-6266764236), left out of #984 as small enough for a follow-up.

1. _kept_name can print a rename git refuses. In templates/assembly-root/scripts/bootstrap.py (_kept_name, the loop near :885 at #984's head), a candidate <twin>-kept is skipped only when a held branch FOLDS to the same name. A branch stored UNDER the candidate is not checked.

• Scenario: the leg is detached at the pin, Main is packed and holds a commit, and a branch Main-kept/x exists.
• The printed line is git -C spec branch -m Main Main-kept. Pasted through /bin/sh, it exits 128: error: 'refs/heads/Main-kept/x' exists; cannot create 'refs/heads/Main-kept'. The next run prints the same command again, so the person has to pick a name themselves.
• On a folding disk, a branch main-kept/x blocks the rename the same way.
• This is false against the docstring's "so the line that says to rename it can be pasted as it is". It is rare, and the line does state the rule, so a person can recover.
• Fix: also skip a candidate when any held name starts with _fold(candidate) + "/", and add a test row for it.

2. No test has a twin whose name contains a slash. rv984b's mutant O3 survives with 63 of 63 passing. O3 makes _folded_spelling return ref.rsplit("/", 1)[-1] instead of ref[len("refs/heads/"):].

• The code is right: with --branch team/main and a packed Team/main, the run is refused; the printed git -C spec branch -m Team/main Team/main-kept works; the next run makes team/main; and Team/main-kept holds the commit.
• Only a test row is missing: a twin with a slash, which kills O3.

Not claimed. Filed by lane openRepoShape-5.

Lane: openreposhape-5 (openRepoShape-5)