AgentCliMessageCapRelief extension

gh-1680: the message-cap self-heal's host relief. The provider refused the request because the message ARRAY exceeded its cap (not the tokens), so the success criterion here is the MESSAGE COUNT, not the token estimate (_relieveOverWindow's no-op test would misjudge a token-healthy transcript). ONE synchronous compaction pass over the live transcript; returns the relieved list for the loop's single retry, or null when the count could not be reduced (nothing hideable — the retry then rides the coalesced payload alone, and a second refusal surfaces the terminal error).

on

• AgentCli