reduce logging

[maks-ivanov]

Note

Medium Risk
Changes pi --mode json output semantics from a full event stream (including session header and streaming updates) to a filtered transcript, which may break existing consumers parsing the previous JSONL event format.

Overview
pi --mode json now emits a compact JSONL transcript instead of streaming all session events, writing only agent_start, completed user messages (message_end), completed assistant turns (turn_end message), and agent_end.

Docs and CLI help text are updated to reflect the new JSON transcript mode, and tests are expanded to assert that non-transcript events (e.g., turn_start, message_update) are no longer printed.

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