skill(doublecheck): run in forked subagent context

[wisenbergg]

The doublecheck skill runs a 3-layer verification pipeline (claim extraction, web source search, adversarial review) that pulls in substantial reference material. Running it inline pollutes the main chat context with intermediate fetches and lowers the quality of follow-up responses.

Setting context: fork isolates execution in a dedicated subagent. The structured verification report still returns to the main chat, but the intermediate noise stays out of it. Same pattern already used by microsoft-skill-creator.

Changes: 1-line frontmatter addition in:

• skills/doublecheck/SKILL.md
• plugins/doublecheck/skills/doublecheck/SKILL.md

[github-actions]

⚠️ This PR targets main, but PRs should target staged.

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Please close this PR and re-open it against the staged branch.

You can change the base branch using the Edit button at the top of this PR,
or run: gh pr edit 1578 --base staged

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[aaronpowell]

It looks like you've incorrectly branched from the main branch not staged, and as a result all the materialised plugins are included in this PR.

You can attempt to fix this with a rebase:

git fetch origin staged
git rebase --onto origin/staged origin/main <branch name>
git push --force-with-lease

If that does not resolve it, you can run npm run plugin:clean which will delete the materialised plugins and you can commit that change.