docs: finalize Signal Check editorial style guide (closes #337) (#412)

* docs: finalize editorial style guide v1.0 (closes #337) Add version number and issue reference to the editorial style guide. All acceptance criteria for #337 are met: - Two-host format defined (Curator + Skeptic) - Segment order locked (7 segments) - Source-backed claims via claim ledger requirement - AI voice disclosure in first 60 seconds - Safety boundaries and prohibited content rules - 1,200-1,700 word target - Distinctiveness from Hard Fork/NYT - Quality checklist for pre-publish validation Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> * docs: rename 'Closes' to 'Issue' in style guide header Addresses review comment — only PRs/commits close issues on GitHub, so the header field should read 'Issue' for consistency with other docs. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> --------- Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>