Adds --watch / -w to coder exp sync timeline (PR 3 of the Script Coordination timing stack, based on #27150).

Summary

• The one-shot graph renderer is refactored into a stateful timelineRenderer whose renderEvents consumes a growing event log incrementally, skipping already-seen events by sequence number. Lane assignment, active-lane tracking, and readiness state persist across calls, so streamed rows are byte-identical to a one-shot render of the same log. renderTimeline (one-shot) is now a thin wrapper.
• --watch renders the events recorded so far, then polls the agent every second and prints only newly produced rows until interrupted. Interruption (Ctrl+C) exits cleanly.
• --watch is text-only; combining it with --output json returns an error.

Testing

• Renderer: chunked/overlapping incremental feeding asserted byte-identical to one-shot rendering, including across a connector-row boundary.
• CLI: pty-based integration test covering pre-watch backlog rendering, live streaming of new events, and clean exit on interrupt; error-path test for --watch --output json; regenerated help golden.

Generated by Coder Agents on behalf of @SasSwart.