feat(ci): Backport CI Workflow

[marioevz]

Description

Adds a label-driven backport workflow so a merged PR can be cherry-picked onto another long-lived branch automatically (e.g. benchmarks/amsterdam), without blocking mainline development.

.github/workflows/backport.yaml does this:

• Selective, via a label. The workflow acts only on PRs carrying a backport <branch> label (e.g. backport benchmarks/amsterdam); the target branch is read from the label itself. PRs without such a label are untouched.
• Fires on merge. Triggered on pull_request_target (closed, labeled) and gated by merged == true, so the label can be applied before or after merge but a backport only happens once the PR is actually merged.
• Fails safe on conflict. On a clean cherry-pick it opens a PR against the target branch titled [backport <branch>] <original title>. On conflict it aborts and comments the manual cherry-pick steps on the source PR — it never force-pushes and never leaves a partially-applied branch behind. Since benchmarks/amsterdam is deliberately divergent, conflicts are expected to be common and are handed to a human by design.
• Skips non-squash merges. merge_commits: skip means the occasional PR that landed as a merge commit is skipped rather than failing the run; the repo squash-merges, so each PR is a single, cleanly-pickable commit.

Operational prerequisites (out of band, not in this PR):

• Create the backport benchmarks/amsterdam label in the repo. Without a matching label the workflow is a no-op.
• Optionally add a BACKPORT_TOKEN secret (a PAT or GitHub App token with contents:write + pull_requests:write). The workflow uses it when present and falls back to GITHUB_TOKEN otherwise. This matters because a PR opened with the default GITHUB_TOKEN cannot trigger downstream CI, so backport PRs to the divergent benchmarks/amsterdam branch would otherwise land without test runs until re-triggered manually.

Usage: add the backport benchmarks/amsterdam label to a PR (e.g. gh pr edit <num> --add-label "backport benchmarks/amsterdam"); on merge the bot opens the backport PR or comments if it cannot apply cleanly.

Related Issues or PRs

N/A.

Checklist

• Ran fast static checks to avoid CI fails, see Code Standards & Verifying Changes: just static
• PR title has the form <type>(<area>): <title>, where <type> and <area> come from an appropriate C-<type>, respectively A-<area>, label. The title should match the target squash commit message.

Cute Animal Picture

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.49%. Comparing base (3ae3d66) to head (6262198).
⚠️ Report is 11 commits behind head on forks/amsterdam.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           forks/amsterdam    #3262      +/-   ##
===================================================
+ Coverage            93.42%   93.49%   +0.07%     
===================================================
  Files                  625      625              
  Lines                37032    37032              
  Branches              3385     3385              
===================================================
+ Hits                 34597    34623      +26     
+ Misses                1666     1653      -13     
+ Partials               769      756      -13     

Flag	Coverage Δ
unittests	93.49% <ø> (+0.07%)	⬆️

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[LouisTsai-Csie]

When creating this PR, there would be no label an author, please consider:

add_author_as_assignee: true
copy_labels_pattern: '^[AC]-'