Grafana PR deployment annotation

[rossme]

Add Grafana annotation for PR deployments to visualize deployment times per branch.

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Note

Introduces PR-aware deployment annotations in generated Grafana dashboards.

• Adds PR Deployments annotation built from branch name or override via DIFFDASH_PR_DEPLOY_ANNOTATION_EXPR
• Refactors annotations: build_annotations(signal_bundle) now includes both Deployments and optional PR-scoped annotation; extracts helpers deployment_annotation, pr_deployment_annotation, pr_deployment_expr, and escape_promql_label
• Updates CLI help and README to document DIFFDASH_PR_DEPLOY_ANNOTATION_EXPR
• Adjusts tests and golden fixture to validate the new annotation and expression

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[cursor]

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