fix(token-usage): reconcile one flagged session per blocking job

The deferred reconcile pass ran as a single spawn_blocking job that
looped through every flagged session with its throttle pauses inside,
so the run loop could not service notifications, drains, or shutdown
(and could not even set shutdown_flag to cut the sleeps short) until
the whole flag set settled.

The pass now reconciles one session per blocking job under the same
select-on-shutdown pattern process_task uses, and returns to the run
loop as soon as notify or drain traffic arrives — the flags are
durable, a notify task's inline reconcile settles them anyway, and
whatever remains resumes at the next backlog drain or sweep tick.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>