PSIRTSUPT-20625: add provenance check to auto-merge flow

[rhybrillou]

Description

Add comprehensive dependabot provenance check for dependabot auto-merge flow.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Central YAML (base), Organization UI (inherited)

Review profile: CHILL

Plan: Pro Plus

Run ID: f51824c5-9b0d-4594-aa8b-a9597990b639

📥 Commits

Reviewing files that changed from the base of the PR and between 91ca263 and 3bcb645.

📒 Files selected for processing (1)

• .github/workflows/auto-merge.yml

📝 Walkthrough

Walkthrough

The auto-merge workflow now verifies Dependabot actor identity, pull request author type, head branch naming, and head commit signature before invoking the auto-merge action.

Changes

Dependabot provenance verification

Layer / File(s)	Summary
Add provenance verification gate .github/workflows/auto-merge.yml	Adds a gh api-based shell step that checks the actor, author type, branch prefix, and verified commit signature, failing with an error before auto-merge proceeds.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the main change: adding a provenance check to the auto-merge flow.
Description check	✅ Passed	The description matches the template and briefly explains the change.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yann/PSIRTSUPT-20625

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In @.github/workflows/auto-merge.yml:
- Around line 12-18: Prevent shell injection in the workflow’s provenance checks
by removing direct GitHub expression interpolation for attacker-controlled
pull-request metadata, including the branch name near the actor validation.
Expose required expression values through env entries, then reference them only
as quoted shell variables in the run script while preserving the existing
validation and rejection behavior.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Central YAML (base), Organization UI (inherited)

Review profile: CHILL

Plan: Pro Plus

Run ID: 074b0a97-7726-417f-a105-f3ada19d77e5

📥 Commits

Reviewing files that changed from the base of the PR and between afb7e0b and 91ca263.

📒 Files selected for processing (1)

• .github/workflows/auto-merge.yml