feat(work-items): add /work-items:measure-delivery with the five DORA delivery metrics #11874

Problem

Nothing reports delivery performance as a team trend.

Origin: Q521; skill name measure-delivery (owner, 2026-10-20); failed deployment = a revert of a
merged PR (owner, 2026-10-21).

Fix

A new read-only skill, /work-items:measure-delivery, reporting the five DORA delivery metrics
(deployment frequency, change lead time, change fail rate, failed deployment recovery time,
deployment rework rate) as a team trend, never per person. Deploy signal is a repository setting, default merge to the default
branch. A merge later reverted counts as a failed deployment. Provider reads go through the bound
tracker adapter or a declared-provider surface, never gh in the skill body (ADR 0060). Per-issue
stage times are a separate item.

Done when

• plugins/work-items/skills/measure-delivery/SKILL.md exists (absent at base 899a46af84).
• The schema declares the deploy-signal key with default merge-to-default-branch.
• Script tests (*.test.sh or test_*.py) over a fixture of merges and reverts check each metric's value; they pass under scripts/run-test-file.sh.
• No output names a person; bash plugins/skill-quality/scripts/check-skill.sh plugins/work-items/skills/measure-delivery exits 0 and bash scripts/check-changelog-fragments.sh passes with a work-items fragment.

Parent: #7578