fix: declare grafast as peer dependency for pg-many-to-many via packageExtensions

[pyramation]

Summary

@graphile-contrib/pg-many-to-many does require("grafast") at runtime but doesn't declare it as a dependency or peerDependency. Under pnpm's strict module resolution, this can cause Cannot find module 'grafast' errors when node_modules is in a stale state (e.g. after switching branches or partial installs).

Adds a pnpm.packageExtensions entry to the root package.json declaring grafast as a peer dependency of @graphile-contrib/pg-many-to-many, ensuring pnpm always links it correctly.

Note: The lockfile was regenerated from scratch, so the diff includes YAML formatting normalization (multi-line resolution: blocks → single-line). No intentional version changes.

Review & Testing Checklist for Human

• Verify lockfile regeneration didn't silently change dependency versions. The diff is large due to YAML formatting normalization. Spot-check a few key packages (grafast, postgraphile, graphile-build, graphql) to confirm versions are unchanged.
• Test the fix locally: rm -rf node_modules && pnpm install, then run pnpm run generate in sdk/constructive-sdk to confirm grafast resolves. This was the original error path.
• Verify CI build passes — the build is the primary signal that nothing broke.

Notes

• On a clean install, grafast is already hoisted to .pnpm/node_modules/ by default and resolves fine. The packageExtensions entry is a defensive measure to make the dependency relationship explicit and prevent resolution failures in edge cases (stale caches, different pnpm versions, etc.).
• The packageExtensionsChecksum in the lockfile changed as expected due to the new entry.

Link to Devin session: https://app.devin.ai/sessions/5d550def7a314c97854ec12fa09dd4ca
Requested by: @pyramation

[devin-ai-integration]

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