Standardize retest workflow across all repositories using plumbing

[vdemeester]

Current State

• A reusable retest workflow already exists in plumbing (_chatops_retest.yml)
• 5 repositories use the plumbing version (dashboard, cli, pipeline, plumbing, hub)
• 4 repositories have local copies not using plumbing (operator, pruner, mcp-server, catlin)
• Multiple repositories are missing the retest workflow entirely

Proposed Solution

Step 1: Migrate existing workflows

• Update 4 repos (operator, pruner, mcp-server, catlin) to use the reusable workflow from plumbing
• Replace local implementations with a simple workflow_call to tektoncd/plumbing/.github/workflows/_chatops_retest.yml@c9d6729a374829a3486b3b4a3c7c67d8b0926f04

Step 2: Add workflow to remaining repos

• Add retest 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 retest-command

Benefits

• Consistent /retest functionality across all tektoncd repositories
• Easier to update/maintain (change once in plumbing, propagates everywhere)
• Single source of truth for retest implementation
• Better automation for CI workflow management

Repositories Affected

Need migration (4):

• chore: migrate retest workflow to use plumbing reusable workflow operator#3086
• chore: migrate retest workflow to use plumbing reusable workflow pruner#110
• chore: migrate retest workflow to use plumbing reusable workflow mcp-server#95
• chore: migrate retest workflow to use plumbing reusable workflow catlin#74

Need addition (3):

• feat: add retest workflow using plumbing reusable workflow results#1167
• feat: add retest workflow using plumbing reusable workflow chains#1488
• feat: add retest workflow using plumbing reusable workflow triggers#1929