feat: add cherry-pick command workflow

[vdemeester]

Changes

This PR adds the cherry-pick command workflow to cli (addresses tektoncd/plumbing#3004).

What's included:

• New cherry-pick-command.yaml workflow using the centralized plumbing reusable workflow
• Enables /cherry-pick <branch> command for easier backports to release branches
• CLI already has slash.yml configured to use plumbing's slash routing, so no changes needed there

Benefits:

• Streamlines cherry-picking to release branches with simple slash command
• Uses centralized, well-tested logic from tektoncd/plumbing
• Consistent behavior with other tektoncd repositories
• Reduces manual cherry-pick errors

Usage: Comment /cherry-pick release-v0.42.x on a merged PR to automatically create a cherry-pick PR to that branch.

Submitter Checklist

• [x] Includes tests (N/A - workflow file only)
• [x] Includes docs (comprehensive inline documentation in workflow file)
• [x] Commit messages follow commit message best practices

Release Notes

NONE

[vdemeester]

/kind misc

[pratap0007]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pratap0007

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• ~~OWNERS~~ [pratap0007]

Approvers can indicate their approval by writing /approve in a comment Approvers can cancel approval by writing /approve cancel in a comment

[tekton-robot]

The following Tekton test failed:

Test name	Commit	Details	Required	Rerun command
check-github-tasks-completed	965832c966663e352e8dac0d557d4537a7b1230a	link	true	/test check-github-tasks-completed

[pratap0007]

/retest