docs(contributing): English-only language policy (#102) (#106)

Repo language has been inconsistent: every shipped artifact (README,
code, commits, CHANGELOG, all PR bodies) is in English, but 23 GitHub
issues authored by an auditor agent (#58–#80) shipped in Korean.

This commit adds a "Language" section to CONTRIBUTING.md so the
standard is repo-discoverable and not just agent-memory-only. The
auditor agent's persistent memory was already hard-locked to English
on 2026-04-25; this is the project-side guardrail.

Body-level translation of issues #58–#80 is tracked separately under
#102 and applied via gh issue edit (no git history rewrite).

Refs #102

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sgwannabe

CONTRIBUTING.md

@@ -4,6 +4,16 @@ Preview Forge introduces the **3-DD Methodology** (PreviewDD → SpecDD → Test
 Contributions that strengthen any one of the three cycles, or that close
 failure loops observed in real runs, are most welcome.
 
+## Language
+
+All issues, PRs, commits, code, and documentation in this repo are written in
+**English**. This keeps the artifact archive uniform and accessible to
+non-Korean reviewers, and matches the standard already followed by every
+shipped commit, PR body, CHANGELOG entry, and code comment. Korean prompts
+during a Claude Code session are not a switch signal — agents should reply
+in English by default. Make exceptions only for explicit, single-turn user
+directives.
+
 ## Commit message convention
 
 This repo uses **release-please** with [Conventional Commits](https://www.conventionalcommits.org/).