Fix Pi RPC completion handling and multi-turn usage accounting #638

Context

CR's Pi RPC adapter stops scanning at agent_end in internal/llmadapters/pi_rpc.go. Pi 1.0 documents agent_settled as the end of automatic work: retries and recovery can follow agent_end. Usage currently flows through overwrite-style mergeUsage, losing earlier completed turns.

Observed baseline and hypothesis

• Environment: local Pi 1.0.0, isolated local mock provider; no paid model or Git-provider writes.
• A mocked HTTP 503 produced agent_end with willRetry: true, then a successful retry and agent_settled. Replaying the captured stream through CR's scanner returned empty structured output rather than the successful JSON: 0/1 recovered answers.
• A synthetic two-turn stream reported 100 + 200 input tokens; CR retained 200 rather than 300.
• Existing Pi adapter tests, including installed-runtime extension preflight, pass. They do not cover these contracts.
• Hypothesis: honoring session settlement and summing authoritative completed-message usage preserves successful recovery and accurate spend without changing review safety.

Acceptance

• Add failing regression tests before production changes.
• Consume through agent_settled, including retries/recovery; do not close stdin at a recoverable agent_end.
• Preserve terminal provider error/abort diagnostics; reject truncated or unsettled streams rather than accepting stale/partial output.
• Sum completed assistant-turn usage once, including cache/cost fields; do not add cumulative streaming updates or duplicate agent_end.messages snapshots.
• Preserve tool allowlists, diff evidence, log caps, deadlines and process-group cancellation.
• Update helper fixtures and document the supported completion contract/version implications.
• Run targeted adapter tests, full Go suite and lint.

Success and rollback

Target: 1/1 recovered successful answer, exactly 300 input tokens for the two-turn fixture, no double counting or safety regression. Validation is local/synthetic, not a deployed outcome. Roll back by reverting the adapter change if runtime smoke tests regress completion or cancellation.

References

• https://github.com/earendil-works/pi/blob/main/packages/coding-agent/docs/rpc.md
• https://github.com/earendil-works/pi/blob/main/packages/coding-agent/docs/json.md

Pi RPC remains the integration. Native session persistence and Pi Durable are out of scope.