fix(tabs): clearer send_to_tab context to stop busy-wait + wrong-recipient replies

Two behavioral problems observed once the tools were usable:

1. The SENDER busy-waited for a reply (ran 'sleep 20' / polled) instead of
   ending its turn. Tool description, the delivery result text, and the
   system-prompt one-liner now say plainly: do not sleep/poll/run commands
   to wait; a reply arrives on its own in a later turn (or via read_tab in a
   future turn); keep working if there's other work, else end your turn.

2. The RECIPIENT replied to its OWN user in plain text instead of routing the
   answer back through send_to_tab. The provenance wrapper now states the
   message is from another AGENT (not your user), and that to reply you must
   use send_to_tab addressed to the sender's handle — and only if asked, since
   it may just be context. A plain text answer reaches only your own user.

Tests updated for the new wording.

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