fix: rename provider id to github-copilot-amplifier for exact cost

[David Koleczek (DavidKoleczek)]

Bug

When opencode uses amplifier-agent as an OpenAI-compatible provider through this bridge, the session cost opencode displays is overstated. amplifier-agent's own cost_usd is Anthropic-exact, but opencode ignores it and re-derives cost from token counts times the static per-model rate table this bridge writes. Under Anthropic prompt caching that is wrong: cache-read tokens get folded into the input bucket at the full input rate, and cache-creation tokens are dropped (recorded as 0), producing a systematic overcharge.

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

Fix

opencode only enables raw-usage passthrough (includeRawChunks) when the provider id contains the substring github-copilot. That passthrough is what lets amplifier-agent's per-turn copilot_usage.total_nano_aiu reach opencode, which then overrides its token-times-rate estimate with the exact value.

• Rename DEFAULT_PROVIDER_ID from amplifier to github-copilot-amplifier to turn that path on. The provider's display name stays Amplifier, so the model picker is unchanged for users.
• On config rewrite, drop any stale provider block previously written under a different id but the same baseURL. Without this, upgrading an existing config leaves a duplicate Amplifier provider (the old amplifier id plus the new one).

Only the loose github-copilot substring is matched. The exact-match Copilot machinery (OAuth, device-flow login, request-body rewriting) is gated on === "github-copilot" or the Copilot npm loader, neither of which this hits.

This depends on the companion change in amplifier-agent (emit copilot_usage on the terminal SSE chunk). 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).
• After the stale-cleanup change, re-running prepare on a config that had both ids leaves exactly one provider (github-copilot-amplifier).

The session completed with no 4xx. ruff check and ruff format --check pass on the changed file.

[David Koleczek (DavidKoleczek)]

Companion PR (required together): microsoft/amplifier-agent#96