Multi-step tool calls overwrite token counts instead of accumulating (output, reasoning, cache.write lost)

Bug

In multi-step tool-call assistant messages, processor.ts overwrites assistantMessage.tokens on each step-finish event instead of accumulating additive fields. Only the last step's output, reasoning, and cache.write survive.

Root cause

packages/opencode/src/session/processor.ts step-finish handler:

ctx.assistantMessage.cost += usage.cost     // accumulates
ctx.assistantMessage.tokens = usage.tokens  // overwrites!

Impact

For an assistant message with N tool-call steps, the message-level tokens only reflect the last step:

• Context metrics (session context meter) under-report token usage for multi-step messages
• compaction.isOverflow checks use lastFinished.tokens (prompt.ts), so underestimating tokens delays auto-compaction and makes the next request more likely to hit the provider context limit

Notes

• Follow-up to #21913, which was auto-closed for inactivity (bot comment says to open a new one if still relevant). The bug still exists on the latest dev.
• Earlier fix attempts (#22077, #22032, #10215) were closed unmerged by automated cleanup, not by technical review.

Expected behavior

Output, reasoning, and cache.write tokens should accumulate across steps (like cost already does). Input and cache.read should keep the final step's values, since the final step's request already includes all prior steps.