Fix error wrapping: use %w for primary child error in tmux action

In runInTmuxChild, the primary failure is the child runFn error; the
echo-through failure is secondary context. Wrap the primary error with
%w (keeping copyErr as %v, since fmt.Errorf allows only one %w) so
errors.Is/As work across the chain.

Audited all other fmt.Errorf %v/%s sites in internal/ and cmd/: they
format strings, ints, status codes, or an []error aggregate (syncer),
none of which is a single primary error to unwrap, so they stay as-is.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions