Add build agent trace foundation

[alandotcom]

This change adds a provider-neutral trace for build-agent turns. The private eval harness can now inspect full model-facing tool data and exact graph revisions, while production logs retain aggregate metrics and exclude workflow and tool payloads.

Review guide

Start with packages/core/src/backend/agent/trace.ts and packages/core/src/backend/agent/chat.ts for the trace contract and step lifecycle. Then review packages/core/src/backend/services/agent/chat.ts for aggregate logging and capacity ownership. The RPC adapter in packages/core/src/backend/rpc/router.ts keeps the producer fiber inside the application runtime and binds cancellation to the request signal and iterator cleanup.

What changed

• Each model step records tool calls, model-facing results, provider errors, finish reasons, usage, and immutable graph revisions.
• Incomplete provider finishes fail the turn, and provider failures produce one safe browser error.
• Eval artifacts retain the full JSON-safe trace. Production records contain counts, timing, status, usage, and finish reasons.
• Streaming uses a bounded mailbox with managed fiber ownership, backpressure, and interruption-safe capacity release. The public AgentStreamPart contract remains unchanged.

Validation

• pnpm run type-check
• pnpm run lint
• pnpm run test: 3,567 passed; 37 PostgreSQL tests skipped because no test database was configured
• pnpm run build
• pnpm run knip
• pnpm run skills:validate
• pnpm run check
• pnpm run evals: 28 of 28 model-backed runs passed on the unchanged baseline and on this branch
• Final Effect review and thermonuclear review approved the branch

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commit: 552f213

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Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
🔒 Security Review	✅ Completed 2026-09-03T19:33:20.589499Z	80cc93c	Draft marked ready

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