[BUG] SubagentHandback refused in bypassPermissions; the report is hidden from the parent

Summary. On 2.1.284, when a subagent runs in bypassPermissions mode, its SubagentHandback call is refused. The parent receives no report, and the subagent's unsent text is not shown, so the report is lost.

Evidence (one session, 2.1.284):

• bypassPermissions: 8 of 8 SubagentHandback calls were refused on 2026-09-29 (2 subagents, 4 attempts each).
• Auto mode: 254 delivered. Plan mode: 16 delivered.
• Not our hooks: no PreToolUse hook denied these calls; our hook log has 0 entries for this tool.
• Binary strings: the 2.1.284 binary contains "Only the auto-mode classifier can allow" and "Its unsent text is not shown".
• The obvious fallback fails: telling the subagent to put its report in its final message does not help, because that text is the part that is hidden. The parent had to send 9 follow-up messages.

Ask.

• Allow SubagentHandback in every permission mode. It only returns text to the parent.
• If it must be refused, deliver the handback text (or the subagent's final message) to the parent instead of hiding it.

Related: #86451 and #97654 (classifier denials in bypassPermissions).