Clarify writing voice for finished analyses

[MaxGhenis]

PolicyEngine’s writing guidance now tells authors to introduce finished analyses without draft, review, or repair history. It names the sentence patterns to remove: self-referential glosses, argument pointers, empty contrasts, vague references, and unintroduced roles.

The guidance keeps scientific assumptions, uncertainty, limitations, and forecast-registration evidence with the claims they qualify. Corrections, changelogs, PR descriptions, and release records continue to describe changes when those changes are their subject. Authors check mechanism claims against primary sources and reread the whole document after a reviewer flags a recurring pattern.

Validation: 178 tests passed with the policy-calculation example suite excluded; the Claude wrapper built successfully and contains the exact source skill; the skill validator and git diff --check passed. No executable examples, bundles, package versions, or calculations changed.