task-monitor 2/6: worker PR bodies open with a plain-language ## Summary #274

Part of #272. Can land in parallel with 1/6. It is a prompt change and pays off immediately.

Scope

• On every loadout (claude and kiro, all trackers), the worker prompt requires the PR body to open with ## Summary: two or three plain sentences, written for the user rather than a reviewer, saying what changed and why. Closes … and the existing sections follow it.
• Update the reviewer prompt so a missing ## Summary is a nit.
• Docs: the workflow docs, the steering templates, and the README. CHANGELOG with an "Upgrading: re-run task-init" note.

Done when

A worker launched after task-init writes ## Summary first, and the parity and doc suites pin it on every loadout.