Add fact-grounding and section-organization rules to PR guidelines

[ikuwow]

Summary

• Land items 1 and 2 of retrospective Retrospective: 2026-07-10 業務プロジェクトの Go サービスへの LD flag 制御 shadow mirror 経路追加 PR #280 in claude/skills/git-workflow/pr-guidelines.md

Changes

• Sources and references (PR Body Checklist): operational sections (verification, rollout, rollback, monitoring) may name an external tool or service only after confirming the project actually uses it; rationale attributed to a linked issue / PR must match that source; violations are Must Fix
• Style rules: organize sections by reviewer / operator needs; no property-themed sections ("safety valves", "performance notes") — fold each point into the bullet for the change it qualifies

Background

Retrospective #280 recorded two fact-grounding failures persisting into an approved plan and PR body (a monitoring vendor the project does not use, and rationale misattributed to an unrelated PR), plus a property-themed section that needed a user editorial pass. pr-selfcheck loads this checklist in Step 3, so both additions take effect on the detection side automatically

Item 3 of #280 (hookify rule blocking $() / backtick command substitution in Bash tool calls) involves a machine-local file outside this repository and remains tracked on the issue

Issue: #280

Verification

• pre-commit hooks passed on commit
• Both bullets match the drafted countermeasure text in issue Retrospective: 2026-07-10 業務プロジェクトの Go サービスへの LD flag 制御 shadow mirror 経路追加 PR #280