chore(vendor): import pi 0.85.1

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: d981de1229ef899957bbe968bc8dcda02a21f477

third_party/pi/packages/coding-agent/test/suite/README.md

@@ -0,0 +1,10 @@
+# Coding agent suite tests
+
+Use `test/suite/` for the new harness-based test suite around `AgentSession` and `AgentSessionRuntime`.
+
+Rules:
+- Use `test/suite/harness.ts`
+- Use the faux provider from `packages/ai/src/providers/faux.ts`
+- Do not use real provider APIs, real API keys, network calls, or paid tokens
+- Keep these tests CI-safe and deterministic
+- Do not use or extend the legacy `test/test-harness.ts` path unless a missing capability forces it