Summary

Partially addresses #656. When av sync --prune (or any code path hitting runDelete in internal/git/gitui/prune.go) fails to delete a merged branch for a reason other than worktree ownership, the error message now includes the git stderr instead of the opaque exit status 1.

Why this matters

Issue #656 reports the user-facing error:

error: cannot delete merged branch "more_ci_improvements": git branch: exit status 1

The "cannot delete merged branch" wrapper text traces to line 235 of prune.go, where the fatal-error path formatted the underlying error with %v. Since the error is an *exec.ExitError, %v resolves to exit status 1 — stripping off the stderr git wrote to explain the failure. PR #660 already handled the specific "used by worktree" phrasing by catching that stderr and producing a recovery-oriented message, but any other failure reason still fell into this opaque path.

What changed

internal/git/gitui/prune.go — inside runDelete(), the else branch (non-worktree fatal error) now pulls exiterr.Stderr off the *exec.ExitError (same errutils.As helper the worktree check above already uses) and passes it to the error message:

errMsg := err.Error()
if exiterr, ok := errutils.As[*exec.ExitError](err); ok {
    if stderr := strings.TrimSpace(string(exiterr.Stderr)); stderr != "" {
        errMsg = stderr
    }
}
deletionErr = errors.Errorf("cannot delete merged branch %q: %s", branch.branch.Short(), errMsg)

So the user now sees whatever git actually said rather than the wrapped shell status.

Scope notes

• The existing worktree-error handling (added in Added message for branch deletion error due to worktrees #660) is untouched; its friendlier recovery message still applies to "used by worktree" stderr.
• The detached-HEAD concern from av sync needs to provide a better error message when branch can't be deleted due to worktrees #656's second paragraph is already handled by the unconditional CheckoutInitialState() call after the delete loop; no change needed there.
• Marking this as Part of #656 rather than Closes because the issue's language covers worktree-specific UX that was delivered in Added message for branch deletion error due to worktrees #660, and this PR fills the remaining surface for non-worktree failures.

Testing

go vet ./...           # pass
go build ./...         # pass
go test ./internal/git/... ./cmd/av/...   # pass

Part of #656

This contribution was developed with AI assistance (Codex).