fix(ci): retire broken metadata Jenkins trigger

[KartikP]

Summary

• skip post-merge kickoff for metadata-only language PRs
• remove the unused update_existing_metadata Jenkins helper and CLI
• document that metadata-only merges remain Git-only and add regression guards

Behavior boundary

• metadata-only merges do not trigger Jenkins or mutate database state
• scoring-eligible plugin merges continue to trigger core/job/score_plugins

Validation

• focused dependency-free regression guards
• Python syntax compilation
• workflow YAML parsing
• stale-reference scan
• git diff --check

This intentionally retires a trigger that targeted a Jenkins job which was never connected to the language workflow. It does not add a replacement metadata mutation path.