docs(spec): add Blueprint Specification v1

[pyramation]

Summary

Adds a formal specification document for the blueprint system at docs/spec/BLUEPRINT.md. This is a documentation-only change — no code, no runtime impact.

The spec defines a three-layer architecture for blueprints:

• Layer 1 (Core) — rigid structural types (BlueprintTable, BlueprintField, BlueprintPolicy, BlueprintIndex) derived from DB introspection
• Layer 2 (Registry) — semi-extensible node types (Authz*, Data*, Relation*, View*) driven by the node-type-registry package and its JSON Schema definitions
• Layer 3 (Extensions) — fully open extension points with namespaced BlueprintExtension entries for frontend scaffolding, deployment recipes, agent skills, etc.

Also documents the type generation pipeline (generate-types.ts), validation rules, versioning policy, and a complete CRM example blueprint.

Review & Testing Checklist for Human

• Layer 3 (Extensions) is aspirational, not implemented. The BlueprintExtension type with namespace/kind/version/config does not exist in the codebase yet. Verify you're comfortable with the spec documenting this as a design alongside the already-implemented Layers 1 and 2. Consider whether it should be marked more clearly as "proposed" or "future."
• Verify BlueprintPolicy discriminant field name. The spec and examples use $type for the policy type field. The introspection-derived version uses policy_type (the actual DB column name). Confirm which is canonical and whether the spec/examples are consistent with what construct_blueprint() actually accepts.
• Spot-check the complete CRM example (lines ~400-520) against the actual generated types — particularly RelationBelongsTo params (field_name, nullable) and DataSearch.full_text_search.source_fields shape.

Notes

• Companion to the existing docs/spec/SLICING.md (cross-referenced in the doc).
• No test plan needed — this is pure documentation.

Link to Devin session: https://app.devin.ai/sessions/ce1b2dafd42341bea5d7793b0c05ba6c
Requested by: @pyramation

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✅ Devin Review: No Issues Found

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