Use PR instead of direct push to main

[dalonsoa]

Description

As there are restriction issues when pushing things to main, now the baseline is added to main via a PR, that needs to be approved and merged.

Mind that as the branch to do the PR from is fixed, if we have multiple PR open, the last one to complete its benchmark run will be the one whose baseline benchmark is considered.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc
• Update release notes for the latest release if this PR adds a new feature or fixes a bug
  present in the previous release

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.47%. Comparing base (5e5c403) to head (3dcbe1a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1516   +/-   ##
=======================================
  Coverage   90.47%   90.47%           
=======================================
  Files          61       61           
  Lines        8899     8899           
  Branches     8899     8899           
=======================================
  Hits         8051     8051           
  Misses        523      523           
  Partials      325      325           

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[Copilot]

Pull request overview

This PR changes the CI benchmark-baseline update mechanism so that baseline updates on main are proposed via an auto-generated pull request (rather than a direct push), aligning with main-branch push restrictions.

Changes:

• Replace the “commit & push updated baseline” step with an automated PR created by peter-evans/create-pull-request.
• Add workflow documentation explaining the fixed PR branch strategy, recursion behaviour, and [skip ci] usage.

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