test: cover Pi standalone relay path

[raphant]

Summary

• Add a standalone Pi smoke test with isolated synthetic sessions.
• Verify Pi replay, a persisted child, a failed tool result, and a live JSONL update through SSE.
• Run web, extension, and standalone production builds in pull-request CI.
• Run the Pi smoke against the built standalone package.

Verification

• pnpm test
• pnpm --filter agent-flow test
• pnpm --filter agent-flow lint
• pnpm --filter agent-flow lint:test
• pnpm --dir web exec tsc --noEmit
• pnpm run build:web
• pnpm run build:extension
• pnpm run build:app
• pnpm run test:pi-smoke
• git diff --check

This is the first Pi-specific slice of issue #14.
It does not close the remaining UI and lifecycle test scope.