feat: add response streaming

[fahimmehraj]

Implements #23

Replaces the chat UI's 500ms job polling with live token streaming from the daemon. The OpenCode SDK already emits message.part.delta events; we now route them to running jobs, accumulate them into job state, and push them over a long-lived connection to subscribed clients.

Features

• New job.stream protocol method: opens a stream that yields a snapshot of the job's accumulated text, then live deltas, then a terminal done/error event.
• client.streamJob() async generator on the client.
• Chat UI iterates the stream and renders tokens as they arrive.

Refactors (and why)

• handleRequest is now an async generator. Previously the connection handler had a special-case branch for job.stream that kept the socket open while every other method used a different code path. Making every handler yield 1+ messages unifies the dispatch — the connection handler just for awaits, no branching. New streaming methods cost zero special-casing.
• Job state unified: outputText accumulates throughout the run, not just at completion. Without this, the daemon and the live delta stream are two sources of truth, and a late/reconnecting subscriber can't recover what they missed. Now subscribeJob emits a snapshot of the current accumulated text before live deltas begin, so subscribers always see a complete picture regardless of when they join.

Tests
9 new tests covering snapshot delivery, terminal-state short-circuit, delta accumulation, the extractText fallback when no deltas arrive, end-to-end generator dispatch, late-subscribe ordering (catches lost or duplicated deltas), and end-to-end client.streamJob over a real Unix socket.