fix(http): emit copilot_usage.total_nano_aiu for exact opencode cost

[David Koleczek (DavidKoleczek)]

Bug

When opencode uses amplifier-agent as an OpenAI-compatible provider, the session cost opencode displays is overstated. amplifier-agent's own cost_usd is Anthropic-exact, but opencode never uses it: it re-derives cost from token counts times a static per-model rate table. With Anthropic prompt caching this is wrong in two ways -- cache-read tokens get folded into the input bucket and billed at the full input rate, and cache-creation tokens are dropped (recorded as 0). The net effect is a systematic overcharge.

Reported in microsoft-amplifier/amplifier-support#352.

Fix

Emit a top-level copilot_usage: {total_nano_aiu: round(cost_usd * 1e11)} as a sibling of usage on the terminal stop / tool_calls SSE chunk (_wire.py), derived from the per-turn usage_cost the route already accumulates (chat_completions.py).

opencode reads copilot_usage.total_nano_aiu off the raw SSE chunk and overrides its token-times-rate estimate with total_nano_aiu / 1e11. Because that value is derived from cost_usd, opencode's displayed cost becomes exactly cost_usd. The field is additive; clients that do not read it ignore it.

This is only consumed when opencode enables raw-chunk passthrough (includeRawChunks), which it does only when the provider id contains the substring github-copilot. That is the companion change in amplifier-app-opencode (rename the provider id to github-copilot-amplifier). Both changes are required together; neither has any effect alone.

Verification

Verified end-to-end in an isolated environment (opencode 1.17.15, amplifier-agent) with a 3-turn, cache-heavy session on claude-haiku-4-5:

• Before: opencode recorded $0.04905320 vs amplifier-agent cost_usd $0.03107695 (+57.8%).
• After: opencode recorded $0.03055695 == amplifier-agent cost_usd $0.03055695 (penny-exact, gap eliminated).

The session completed with no 4xx, confirming amplifier-agent tolerates the Copilot request headers opencode injects for github-copilot-substring providers. ruff check and ruff format --check pass on the changed files.

[David Koleczek (DavidKoleczek)]

Companion PR (required together): microsoft/amplifier-app-opencode#15