- add `[background].print_background_mode` (`exit`/`drain`/`steer`) and
`print_max_turns`; when unset, falls back to `keep_alive_on_exit = true`
mapping to `drain`, preserving existing behavior
- core: `Session.handlePrintMainTurnCompleted()` returns `finish`/`continue`;
in `steer` mode the run stays alive so a background-task completion
`turn.steer`s the main agent into a new turn (matching background
subagents), bounded by `print_wait_ceiling_s` and `print_max_turns`
- cli: print driver follows every main turn instead of only the first and
defers `finish()` until the run quiesces or a limit is hit
- plumb `handlePrintMainTurnCompleted` through node-sdk RPC; docs + tests
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