feat(client): add runtime-neutral session client

[christianklotz]

Summary

• add the runtime-neutral @earendil-works/pi-client package with injectable ordered-byte transports
• expose authenticated connection setup, typed client operations, server-owned session handles, detach, reconnect, snapshots, and events
• isolate transport, framing, and handshake lifecycle from client state while guarding synchronous callback reentrancy
• keep raw protocol requests private and expose non-constructible PiSessionHandle values only through create/attach operations
• report listener failures without allowing consumers to corrupt protocol state
• split harness shutdown initiation from settlement so hooks and subscribers cannot deadlock active work
• integrate the package with workspace builds, publishing, type checking, and browser smoke checks

Verification

• npm run check
• client tests: 22 passed
• focused agent harness tests: 33 passed
• protocol tests: 126 passed
• ./test.sh: client, protocol, AI, evals, and TUI passed; agent SQLite and coding-agent child-process suites could not resolve unbuilt workspace dist entries in this fresh worktree, and the unrelated bash timing regression failed

This PR is AI-generated by pi.