Evaluate a [[bootstrap.command]].when expression against the given
worktree. Supports the keyword predicates file_exists:, cmd_exists:,
env_set:, env_eq:, glob_exists:, plus the boolean operators !,
&&, || with conventional precedence (! > && > ||). Unknown
keyword predicates default to true so older configs keep running while
the doctor surfaces them as warnings.
Return every atom string contained in a when expression, dropping
the boolean operators. Callers (e.g. doctor::check_when_predicates)
can then validate each atom independently — w.starts_with(prefix)
on the raw expression misses negated atoms (!env_set:CI) and
unsupported keywords sitting on the RHS of && / ||.
Companion to copy_no_follow for callers that already hold the
payload in memory (e.g. the inline-fallback branch in
[resolve_missing]). Same TOCTOU-closing semantics: dst must
not exist and must not be a symlink, or open() fails.