Show what a waiting thread is waiting on (PR watch, child task, background work)

[johan-acevedo]

Problem. A thread that waits shows, for example, "Waiting · Watching pull request #83". A user asked in the thread: "what is going on? are you waiting or listening to a callback?". From the sidebar you cannot tell:

• what exactly it waits on (a PR watch, a delegated child, a background command, or several);
• what will wake it (checks passing, a review, a child finishing) and whether anything is still running.

The structure already exists: pending background work is modelled in orchestrationV2PendingBackgroundWork.ts, and task_status has a workState.

Idea. Surface that structured reason in the sidebar row, the thread header and t3 thread show. For example: "Waiting on: PR #83 checks (3/5 done) · child 'review' running 6 min", with the wake condition in a tooltip.

Related, not duplicates: #15993 (stale waiting banner) and #22183/#20523 (watches ending silently). A visible wait reason would also make those failures easier to notice. Observed on t3 v0.1.8-nightly.20261106.4247.