fix(metering): accumulate subagent token usage into parent session (SAM-66)

[dembrane-sam-bot]

• What is this change?: Passes the parent session's usage_state into _make_parallel_workers_tool and _make_consult_opus_tool, and calls _accumulate_usage(event, parent_usage_state) on each subagent event inside their asynchronous run loops.
• What did Sam notice that led to this?: Subagent runs for parallel_workers and consult_opus executed as separate sub-runners, and because they did not call _accumulate_usage, all of their output and prompt token usage was entirely omitted from the parent session's accumulators. This hid real token burn and resulted in an output token undercount of up to 5.7x, as documented in SAM-66.
• Tier?: Tier 3 (runtime)
• Confidence?: 100% (passes all 654 runtime and structural tests, including a new unit test verifying parameter signature compliance).

[linear]

SAM-66