fix(skills/update-stack): switch drift gate scan to git diff (catches missing-locally case)

[PierreBrisorgueil]

Summary

• Replaces git ls-files modules lib config iteration (locally-present files only) with git diff --name-only devkit-node/master HEAD (bidirectional diff)
• Adds a second BLOCK arm: files present upstream but absent locally → BLOCK: missing locally (was on upstream) with a restore hint
• Downstream-only files (not in upstream) are still skipped silently
• Updates the Rules bullet to explain why git diff was chosen over git ls-files

Root cause

The old gate iterated git ls-files which only enumerates files that exist in the working tree. A file deleted from a downstream (but still in upstream) was silently skipped — the gate could never detect the "missing locally" case.

Test plan

• Smoke: bash -c '...' on the gate block — exits 0 on clean tree
• Verify the gate runs to 3ter: no drift — OK on a clean downstream after /update-stack
• Manually delete a stack file locally, re-run gate → should BLOCK with missing locally message

Closes #3779

Summary by CodeRabbit

• Documentation
  • Enhanced drift detection in the /update-stack command to identify content mismatches and missing upstream files.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: e275cd9c-3db7-4326-a108-6a531f12bceb

📥 Commits

Reviewing files that changed from the base of the PR and between cdacb9b and ef46adb.

📒 Files selected for processing (1)

• .claude/skills/update-stack/SKILL.md

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Walkthrough

Updated the /update-stack skill's drift-blocking step (3ter) to use bidirectional diff-based scanning instead of locally-enumerated files, detecting both shared-file divergence and upstream-present files missing locally.

Changes

Drift Gate Scan Refactoring

Layer / File(s)	Summary
Drift gate scan logic and documentation .claude/skills/update-stack/SKILL.md	Changed the drift-blocking loop to iterate git diff results restricted to modules/ lib/ config, replaced blob comparison with upstream-vs-HEAD existence checks using git cat-file -e, and updated the Rules bullet describing the scan source and detection of both divergent files and upstream-present missing-locally cases.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related issues

• #3779: Directly addresses the same drift gate fix—switching scan source from local git ls-files to bidirectional upstream comparison to catch upstream-present files missing locally.

Possibly related PRs

• pierreb-devkit/Node#3760: Also modifies the /update-stack "3ter. Block on drift" gate to detect upstream-vs-HEAD drift and missing files.
• pierreb-devkit/Node#3778: Updates the same .claude/skills/update-stack/SKILL.md drift gate to change scan scope derivation and handle missing-locally/divergent file cases.

Suggested labels

Chore

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description covers the essential sections: Summary (what changed, why), Root cause, and Test plan. Most required template sections are absent or incomplete.	Add missing template sections: Scope (modules impacted, risk level), Validation (npm lint/test checklist), Guardrails checks, and fill in any Infra/Stack alignment details if applicable.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically summarizes the main change: switching the drift gate scan to use git diff instead of the previous approach, and highlights the key benefit (catching missing-locally case).
Linked Issues check	✅ Passed	The PR successfully implements the requirements from #3779: switches scan source from git ls-files to git diff, detects files missing locally, and adds appropriate BLOCK messaging with restore hints.
Out of Scope Changes check	✅ Passed	All changes are narrowly scoped to updating the drift gate scan logic in the SKILL.md documentation file, directly aligned with issue #3779 requirements.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/drift-gate-scan-source-flip

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[Copilot]

Pull request overview

Updates the /update-stack skill’s “3ter: Block on drift” gate to detect stack-file drift using a symmetric upstream-vs-local diff, including the previously undetectable “present upstream but missing locally” case.

Changes:

• Switches the drift scan source from git ls-files (local-only enumeration) to git diff --name-only devkit-node/master HEAD over modules/, lib/, and config/.
• Adds an explicit BLOCK path for “missing locally (was on upstream)” with a restore command hint.
• Updates the documented rule to explain why git diff is used (bidirectional change detection).