Implement new translation workflow and restructure system prompt

[re moved the 'Created using GitLab Duo' . Adding my human description[

Problem

The system prompt currently has one translation workflow. We need routing logic that determines workflow type before loading resources:

1. File move - no translation needed, just move/rename files to match English structure
2. New translation - target file doesn't exist or has English placeholders, translate from scratch
3. Update existing translation - target file exists with reviewed translations, translate only what changed

This builds on @laurenbarker decision tree (#19 (comment 3053020236)), @mjsibanez's synchronization workflow testing gitlab-com/marketing/digital-experience/about-gitlab-com!3813 (comment 3052138489), and the prototype in !44 (closed)

What needs to be done

• Add workflow routing as step 1 in system prompt (before resource loading)
• Create specifications/workflows/update-existing-translations-workflow.md with tested (or assumed, if was not tested) process
• Add file move workflow section