feat(methodology): Rule 10 — Output language English (Layer-0) (#107)

User-facing output from any of the 144 plugin agents (AskUserQuestion
modal labels, console prose, status messages, Gate H1/H2 modals,
generated artifacts the user reads directly) MUST be in English. Korean
prompts from the user are not a switch signal — agents reply in English
by default. Single-turn override only via explicit per-turn directive.

Why: the repo's archive (commits, PR bodies, code, schemas, root
CHANGELOG) is English-only per CONTRIBUTING.md "Language" section. A
bilingual user surface breaks searchability for non-Korean reviewers
and contradicts the documented policy.

Scope: this rule constrains the *output the agent emits to the user*,
not the language of the agent's own markdown prompt (which may stay in
Korean for maintenance — many existing agent prompts are Korean).

Enforcement: Layer-0 rule, propagated to all 144 agents via
`@methodology/global.md`. factory-policy.py does not parse
natural-language output, so enforcement is by agent compliance. A
future hook may scan AskUserQuestion payloads for Hangul code points
and warn — tracked separately.

Side effects:
- Updates section title from "7개 비협상 규칙" to neutral
  "Layer-0 비협상 규칙" (count was already stale — Rules 8 and 9 were
  added later)
- Updates the 불변 원칙 footer to acknowledge Rules 1-10
- No agent file modified — the rule reaches all agents via the
  existing @methodology/global.md include in their frontmatter
- verify-plugin.sh: still 58/0

Refs #102 (closing the gap that #102 didn't cover — endusers' actual
plugin output language)

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

Author: sgwannabe

plugins/preview-forge/methodology/global.md

@@ -6,7 +6,7 @@
 
 ---
 
-## 7개 비협상 규칙 (Non-negotiable Rules)
+## Layer-0 비협상 규칙 (Non-negotiable Rules)
 
 ### Rule 1 — Gate 없이 진행 금지
 - PreviewDD → SpecDD 전환은 Gate H1 (인간 디자인 승인) 없이 불가
@@ -99,6 +99,43 @@ export PF_DRIFT_BYPASS=1 PF_DRIFT_REASON="SpecDD explicitly expanding to include
 - Gate H1 완료 전 (chosen_preview.json 없음) → no-op
 - 120자 미만 write → no-op (오타 수정 false positive 방지)
 
+### Rule 10 — Output language: English (v1.13+)
+
+All user-facing output produced by any of the 144 agents in this plugin
+**MUST be in English**, regardless of the language the user prompts in.
+This includes (non-exhaustive):
+
+- `AskUserQuestion` modal `question`, `header`, option `label` and
+  `description` fields
+- Console / chat prose responses to the user
+- Status messages, progress updates, standup summaries
+- Modal labels at Gate H1 and Gate H2
+- Error messages surfaced to the user
+- Generated artifacts that the user reads directly:
+  `mockups/gallery-text.md`, `score/report.json` summaries,
+  `runs/<id>/trace.log` user-facing lines
+
+**Why**: this plugin's archive (commits, PR bodies, root CHANGELOG, code,
+schemas) is English-only — see `CONTRIBUTING.md` "Language" section.
+A bilingual user surface defeats searchability for non-Korean reviewers
+and contradicts the documented policy. Korean prompts from the user are
+**not** a switch signal — agents reply in English by default.
+
+**Single-turn override**: only when the user issues an explicit per-turn
+directive such as *"reply in Korean for this answer"*. Once the turn
+ends, the next response returns to English.
+
+**Markdown of the agent files themselves**: the prose **inside**
+`agents/**/*.md` and `commands/*.md` may stay in Korean for maintenance
+(many existing agent prompts are in Korean). What's enforced is the
+*output the agent emits to the user*, not the language of the prompt
+the agent was authored in.
+
+**Enforcement**: this rule is Layer-0; `factory-policy.py` does not
+parse natural-language output, so enforcement is by agent compliance.
+A future hook may scan `AskUserQuestion` modal payloads for Hangul code
+points and warn — tracked separately.
+
 ---
 
 ## 모델 · effort 강제 정책
@@ -153,4 +190,4 @@ Per-call payload cap: 1-4 questions per AskUserQuestion call (Claude Code tool s
 
 ## 불변 원칙
 
-이 문서는 plugin v1.0.0 기준 7 rules를 정의합니다. v2.0.0 이전까지 **추가만 가능, 수정·삭제 불가**. v2.0.0에서 breaking change가 있을 경우에도 각 규칙의 의도는 유지되어야 합니다.
+이 문서는 plugin v1.0.0 기준 7 rules로 출발하여 점진적으로 확장되었습니다 (현재 Rule 10까지). v2.0.0 이전까지 **추가만 가능, 수정·삭제 불가**. v2.0.0에서 breaking change가 있을 경우에도 각 규칙의 의도는 유지되어야 합니다.