ci: PR contamination prevention guard

[diberry]

Adds:

• Diff Size Guard job in squad-repo-health.yml that warns when single-commit PRs touch 30+ files (likely contamination from stale branches)
• Branch contamination prevention rules in copilot-instructions.md (safe branching, no git reset --soft, safe squashing)
• Diff verification checklist item in PR template

Prevents the recurring issue where git add -A on stale branches contaminates PRs with unrelated changes.

Repo-health PR — no product code changes.

[github-actions]

🛫 PR Readiness Check

ℹ️ This comment updates on each push. Last checked: commit 4fa0d5b

⚠️ 3 item(s) to address before review

Status	Check	Details
❌	Single commit	2 commits — consider squashing before review
✅	Not in draft	Ready for review
✅	Branch up to date	Up to date with dev
❌	Copilot review	No Copilot review yet — it may still be processing
✅	Changeset present	Changeset file found
✅	Scope clean	No .squad/ or docs/proposals/ files
✅	No merge conflicts	No merge conflicts
✅	Copilot threads resolved	0 active Copilot thread(s) resolved (3 outdated skipped)
❌	CI passing	14 check(s) still running

---

This check runs automatically on every push. Fix any ❌ items and push again.
See CONTRIBUTING.md and PR Requirements for details.

[github-actions]

🟡 Impact Analysis — PR #825

Risk tier: 🟡 MEDIUM

📊 Summary

Metric	Count
Files changed	5
Files added	1
Files modified	4
Files deleted	0
Modules touched	2

🎯 Risk Factors

• 5 files changed (≤5 → LOW)
• 2 modules touched (2-4 → MEDIUM)

📦 Modules Affected

ci-workflows (3 files)

• .github/PULL_REQUEST_TEMPLATE.md
• .github/copilot-instructions.md
• .github/workflows/squad-repo-health.yml

root (2 files)

• .changeset/contamination-guard.md
• CONTRIBUTING.md

---

This report is generated automatically for every PR. See #733 for details.

[github-actions]

🏗️ Architectural Review

⚠️ Architectural review: 1 info.

Severity	Category	Finding	Files
ℹ️ info	template-sync	Template files changed in .github/workflows/ but not in other template locations. If these templates should stay in sync, consider updating the others too.	Changed: .github/workflows/, Unchanged: templates/, .squad-templates/, packages/squad-cli/templates/

---

Automated architectural review — informational only.

[Copilot]

Pull request overview

Improves contributor guidance to reduce missing changesets by adding prominent reminders in the PR template and expanding Copilot repository instructions around changesets and PR hygiene.

Changes:

• Added a “Quick Check” changeset reminder and an additional diff-verification item to the PR template checklist.
• Added branch hygiene guidance (“Branch Contamination Prevention”) to Copilot instructions.
• Added PR scope rules and a detailed “Changeset Requirement” section (including examples and an escape hatch) to Copilot instructions.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
.github/PULL_REQUEST_TEMPLATE.md	Adds a prominent changeset reminder and an extra checklist item to verify staged diff contents.
.github/copilot-instructions.md	Adds new sections for branch hygiene, PR scoping rules, and changeset requirements with examples.

[tamirdresher]

✅ LGTM — Warning-only diff guard is the right approach. Heuristic (≤2 commits + >30 files) catches the real contamination pattern without blocking legitimate large PRs. Branch hygiene rules in copilot-instructions and PR template checkbox are good additions.

[tamirdresher]

✅ LGTM — Warning-only diff guard catches contamination. Branch hygiene rules and PR template checkbox are good additions.