fix: generate root barrel file in multi-target codegen

[pyramation]

Summary

generateMulti() creates per-target barrel files (admin/index.ts, auth/index.ts, …) but never generated the root src/index.ts that re-exports all targets as namespaces. The SDK's pregenerate script deletes src/index.ts via rimraf, and codegen never recreated it — so after any schema propagation run, @constructive-io/node fails to build:

src/index.ts(31,15): error TS2307: Cannot find module '@constructive-io/sdk'

Fix: Add generateMultiTargetBarrel() to the codegen barrel module, and call it from generateMulti() after all targets complete. It writes:

export * as admin from './admin';
export * as auth from './auth';
export * as objects from './objects';
export * as public_ from './public';   // reserved-word alias

The output root is derived from the first successful target's output path (path.dirname(firstOutput)).

Review & Testing Checklist for Human

• Verify path.dirname(firstOutput) is correct for your codegen configs — this assumes all targets share the same parent directory (e.g. ./src/admin, ./src/auth → ./src). If any consumer configures targets with divergent roots, the barrel would land in the wrong place.
• Confirm the generated barrel matches the old hand-written src/index.ts — specifically that export * as public_ from './public' is produced (the _ suffix for JS reserved words). Run pnpm generate in sdk/constructive-sdk and diff the output against what was on main before.
• Re-run the schema-propagation workflow (or simulate it) end-to-end to confirm @constructive-io/node builds after codegen. Local pnpm build passed on this branch, but CI codegen uses a different path (the hub workflow).

Notes

• The JS_RESERVED set is comprehensive but static — covers ES2015+ reserved words plus future reserved words in strict mode.
• No unit test added for generateMultiTargetBarrel(). Consider adding one if the codegen test suite is easy to extend.
• This does not address the separate Cannot find module 'grafast' error seen when running generate-sdk.ts locally — that's a pnpm peer-dependency hoisting issue with @graphile-contrib/pg-many-to-many and is unrelated.

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

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