feat: add @pgpmjs/migrate-client workspace package for typed Migrate API access

[pyramation]

feat: add @pgpmjs/migrate-client workspace package

Summary

Adds a new workspace package at sdk/migrate-client/ that provides a typed GraphQL ORM client for the Constructive Migrate API (db_migrate schema). This package will be consumed by pgpm/core to replace ~1,300 lines of handwritten GraphQL boilerplate in the export flow (on the feature/pgpm-export-graphql branch).

Package contents:

• schemas/migrate.graphql — static copy of the db_migrate GraphQL schema from constructive-db
• Generated ORM models: SqlActionModel, MigrateFileModel (findMany/findFirst/create/update/delete)
• Generated mutations: executeSql, runMigration
• scripts/generate.ts — regeneration script using @constructive-io/graphql-codegen (devDep only)

Runtime dependencies are minimal: gql-ast, @0no-co/graphql.web, @constructive-io/graphql-types, graphql. No codegen dependency at runtime.

This PR also adds @pgpmjs/migrate-client as a workspace:^ dependency in pgpm/core/package.json, but no code in pgpm/core imports it yet — the actual usage will land via the export PR on feature/pgpm-export-graphql.

Review & Testing Checklist for Human

• ORM code was copied, not locally generated. The pnpm generate script requires graphile-connection-filter (transitive codegen dep) which wasn't available in the build environment. The committed ORM files were copied from constructive-db's existing generated output. Verify that pnpm generate works in a fully-configured environment and produces identical output.
• Schema drift. sdk/migrate-client/schemas/migrate.graphql is a one-time copy from constructive-db. If the upstream db_migrate schema changes, this file must be manually re-copied and pnpm generate re-run. Consider whether an automated sync is warranted.
• Lock file diff is large but cosmetic. The pnpm-lock.yaml diff is ~90% YAML formatting changes (brace style). The substantive additions are only the sdk/migrate-client importer block and the @pgpmjs/migrate-client link in pgpm/core. Spot-check for unexpected dependency changes.
• Build verification. Run pnpm build from workspace root — sdk/migrate-client and pgpm/core should both compile cleanly. Note: concurrent workspace builds can race (migrate-client's dist gets deleted while core compiles); building migrate-client first avoids this.

Notes

• Link to Devin session: https://app.devin.ai/sessions/2b96fabbdaf4455fa4de78f248020902
• Requested by: @pyramation

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