[DD] add grafana PR deployment annotations

[rossme]

Note

Introduces PR-scoped deployment annotations in Grafana dashboards.

• Grafana renderer now builds annotations from signals: keeps base Deployments and adds PR Deployments filtered by branch (change_set.branch_name) or DIFFDASH_PR_DEPLOY_ANNOTATION_EXPR; adds escape_promql_label
• Updates CLI help and README to document DIFFDASH_PR_DEPLOY_ANNOTATION_EXPR
• Adjusts tests and golden fixture to expect two annotations; includes branch in test bundles
• Minor: add .ruby-version

^{Written by Cursor Bugbot for commit 77e1405. This will update automatically on new commits. Configure here.}