add pnpm dev back

[carterworks]

Description

This restores a root-level pnpm dev workflow after the repo split moved browser package scripts under packages/browser.
It uses concurrently to watch-build @adobe/alloy while starting the browser sandbox, so local SDK work feels like it did before the restructure.

Related Issue

Motivation and Context

The old root pnpm dev entrypoint disappeared in the package split, which made a common local workflow harder to discover and repeat.
This adds the workflow back at the monorepo root in a way that matches the new workspace layout and gives clearer process output.

Screenshots (if appropriate):

Checklist:

• I have signed the Adobe Open Source CLA or I'm an Adobe employee.
• I have made any necessary test changes and all tests pass.
• I have run the Sandbox successfully.
• I have added a Changeset (pnpm changeset) or it is not necessary because this PR is not consumer-facing.

[changeset-bot]

⚠️ No Changeset found

Latest commit: ed97162

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR