The previous fix gated on a verified `transferShell` but still let
OpenSSH hand the snippet to whatever `$SHELL` happens to be on the
remote. On a fish/csh/tcsh host the new gate would accept the host,
then fail anyway because the login shell can't parse `if [ ... ]; then
...` (P1 from PR #3722 review).
Each transfer command (read, write, stat, list) is now wrapped in
`<transferShell> -c '...'` via a shared `wrapInPosixShell` helper, so
the snippet is parsed by the same shell OMP's capability probe verified
can run it. `ensurePosixRemote` returns the verified shell so each
call site can do the wrap. `ssh-executor.ts`'s identical Windows-compat
helper is consolidated onto the same primitive (`buildCompatCommand` /
`quoteForCompatShell` removed).
Stays POSIX-clean across all four call sites; `-c` (not `-lc`) since
the snippets only call absolute builtins and don't need login-profile
setup. Capability *probing* still uses `-lc` to mirror the user env.
Test additions: one case asserts every dispatch starts with
`bash -c '...'` and embeds the original POSIX snippet (read/write/stat/list)
when transferShell is bash and login shell is unknown; another covers
the `sh -c` happy path.
roboomp
207734e915