fix(node-type-registry): grant_privileges type is string[][] not string[]

[pyramation]

Summary

Fixes the JSON schema for RelationManyToMany.grant_privileges which was missing its items definition, causing codegen to produce string[] instead of the correct string[][].

The actual runtime usage is an array of [verb, columns] tuples — e.g. [['select','*'],['insert','*']] — so the schema now specifies items: { type: "array", items: { type: "string" } }.

The regenerated blueprint-types.generated.ts also picks up cosmetic changes from recently merged PRs (#914, #915) that are now on main ($type → policy_type on BlueprintPolicy, section comment updates, minor JSDoc tweaks on BlueprintIndex).

Review & Testing Checklist for Human

• $type → policy_type rename on BlueprintPolicy: The regenerated file renames this field. Verify that all consumers (agentic-db, constructive-db blueprints, construct_blueprint() SQL) already expect policy_type and not $type. If any consumer still uses $type, this is a breaking change.
• grant_privileges correctness: Confirm that construct_blueprint() and secure_table_provision actually consume grant_privileges as a 2D array (array of [verb, columns] tuples). If the server flattens or interprets it differently, the type could still be wrong.

Notes

• The source change is only in relation-many-to-many.ts (6 lines added to the JSON schema items definition). Everything in blueprint-types.generated.ts is regenerated output.
• Discovered while updating agentic-db to node-type-registry@0.6.0 — the M2M relations with grant_privileges: [['select','*'], ...] failed type checking.

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

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