feat(work-items): triage and attend-queue claim an item before writing to it #11878

Problem

Only /work-items:work and /work-items:track start claim. Triage and attend-queue write labels and
comments unclaimed, so two runs can post conflicting results on one item.

Origin: D93, D94, Q515 (one claim protocol for every worker; the claim covers any agent write).

Fix

Any work-items skill run that changes an item on its own initiative (a comment, a label, a sub-item)
claims it first with --phase <kind> (K14) and releases it when it posts its result. Read-only
listings (attention view, stats) never claim. A lost claim (exit 7) skips the item. Claims by the
CI lanes, which write as an App, are not part of this item.

Done when

• The triage and attend-queue skills claim before their first write and release after their last, and state the skip on exit 7.
• The work-items README's claim-protocol section names every claiming skill.
• Eval cases expect the claim before a triage write and no claim for the attention view; python3 plugins/evals/skills/validate/scripts/validate-cases.py plugins/work-items/evals and bash plugins/skill-quality/scripts/check-skill.sh plugins/work-items/skills exit 0, and bash scripts/check-changelog-fragments.sh passes with a work-items fragment.

Parent: #7578