Stream Pi session turns

[c0da-ricon]

Summary

• add a domain-neutral PiSessionStream around an injected Pi-like session backend
• stream ordered assistant text deltas while rejecting empty prompts and overlapping turns
• own unsubscribe, early-consumer abort, explicit abort, failure propagation, and idempotent disposal
• add the Bun/TypeScript test and typecheck foundation to Stein's existing public mise run check

Harvest boundary

This adapts the smallest proven turn-streaming seam from the GHL session-note service rather than copying its product architecture. Structural review also closes two generic edge cases: early consumers abort the active backend turn, and even undefined promise rejections remain failures.

Validation

• clean bun install --frozen-lockfile
• mise run check
  • strict TypeScript typecheck
  • 7/7 deterministic Bun tests
  • 6/6 configured Codebase lints
  • whitespace check
• signed commit hooks reran the configured Codebase lints

The first hosted run exposed that the existing workflow installed mise tools but not the newly introduced package dependencies. Commit abef9da adds the required frozen dependency-install step before the unchanged public check.

Boundary

This PR adds no real Pi SDK factory or model call, HTTP/OpenAI protocol, conversation identity, persistence, attachment policy, prompt, deployment, provider configuration, or GHL behavior.