coop: validate blueprint node references

[tomelm]

Summary

• Validate ${node.<step>.<node>...} references when a blueprint is loaded (LoadBlueprint), so malformed or unknown node references fail fast instead of surfacing as confusing errors mid-session.
• Collect valid step.node (and step.node.request) keys, then check every ${node...} reference in the marshaled blueprint against them.
• Allow integer-indexed reference segments (e.g. an indexed test-request) as valid.

Tests

• TestValidateBlueprintReferences — covers valid references, malformed references, and unknown referenced nodes.
• go test ./pkg/coop/... ./pkg/cmd/coop/... — all green.

Base

Based directly on coop. First of the co-op product-quality follow-ups; agent-quality and blueprint-schema build on top and will be opened as coop advances.