slan-cuan-release: Add default for taskGitRevision param

Without a default, the release service cannot create the PipelineRun
since it has no mechanism to pass taskGitRevision. Default to main.

Assisted-by: Claude Code (Sonnet 4.6)

Author: arewm

pipelines/managed/slan-cuan-release/slan-cuan-release.yaml

@@ -82,6 +82,7 @@ spec:
     - name: taskGitRevision
       type: string
       description: The revision in the taskGitUrl repo to be used
+      default: main
 
     # Extract parameters
     - name: registry-auth-secret