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

[PierreBrisorgueil]

Summary

• Replaces git ls-files src/modules src/lib src/config iteration (locally-present files only) with git diff --name-only devkit-vue/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 #4233

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📥 Commits

Reviewing files that changed from the base of the PR and between 54358f0 and 01240ce.

📒 Files selected for processing (1)

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

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

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[Copilot]

Pull request overview

This PR updates the /update-stack “3ter. Block on drift” gate in .claude/skills/update-stack/SKILL.md to source its scan list from git diff --name-only devkit-vue/master HEAD instead of iterating locally-present files via git ls-files, with the goal of also catching “present upstream but missing locally” cases.

Changes:

• Switches the drift scan input list from git ls-files … to git diff --name-only devkit-vue/master HEAD -- … to include upstream-vs-local deletions.
• Adds an explicit BLOCK branch for “missing locally (was on upstream)” with a suggested restore command.
• Updates the documented Rules to explain why git diff was chosen.