Standardize cherry-pick-command workflow across all repositories using plumbing

[vdemeester]

Current State

• A reusable cherry-pick workflow already exists in plumbing (_cherry-pick-command.yaml)
• Only 2 repositories use the plumbing version (pipeline, plumbing)
• 1 repository has a local copy not using plumbing (operator)
• Multiple repositories are missing the cherry-pick workflow entirely

Proposed Solution

Step 1: Migrate existing workflow

• Update operator to use the reusable workflow from plumbing
• Replace local implementation with a simple workflow_call to tektoncd/plumbing/.github/workflows/_cherry-pick-command.yaml@{digest}

Step 2: Add workflow to remaining repos

• Add cherry-pick workflow to repos that don't have it
• Use the plumbing reusable workflow from the start
• Ensure each repo also has the slash.yml workflow configured to trigger cherry-pick-command

Benefits

• Consistent cherry-pick functionality across all tektoncd repositories
• Easier to update/maintain (change once in plumbing, propagates everywhere)
• Single source of truth for cherry-pick implementation
• Better automation for release branch management

Repositories Affected

Need migration (1):

• chore: migrate cherry-pick workflow to use plumbing reusable workflow operator#3085

Need addition (9):

• feat: add cherry-pick slash command workflow chains#1487
• feat: add cherry-pick command workflow cli#2682
• feat: add cherry-pick command workflow hub#2681
• feat: add cherry-pick command workflow and fix slash.yml mcp-server#94
• feat: add cherry-pick slash command workflow results#1166
• feat: add cherry-pick slash command workflow triggers#1928
• feat: add cherry-pick command workflow and fix slash.yml catlin#73
• feat: add cherry-pick slash command workflow pruner#109
• feat: add cherry-pick command workflow dashboard#4653