feat: add per-PR npm beta releases via beta-release label

[benminer]

Why

There was no way to publish a testable npm build of a specific PR before merging it, so changes could only be validated after they'd already landed on main and shipped to latest.

What Changed

Adds a beta-release.yml workflow: labeling a PR beta-release publishes a Changesets snapshot prerelease of that PR's exact code under a pr-<N> npm dist-tag (reusing the existing scripts/publish-adcp-release.ts release script via ADCP_NPM_TAG), republishing on every push and commenting install instructions on the PR. Promotion is just merging the PR normally — the real latest version is cut fresh by the existing changesets + release.yml flow. Includes a best-effort dist-tag cleanup job (gated on an NPM_DIST_TAG_TOKEN secret still to be provisioned), guards against no-op publishes when a PR is missing a real changeset, and rejects running during Changesets pre-release mode. Docs added to docs/development/NPM-DIST-TAGS.md distinguishing this from the existing long-lived beta-channel pattern in v8.0-beta-plan.md.

Note

NPM_DIST_TAG_TOKEN (for dist-tag cleanup) and npm trusted-publisher registration for beta-release.yml (alongside the existing release.yml entry) still need to be provisioned on npmjs.com before this fully works end-to-end.

[bokelley]

Not merging this into the current release train as-is. main is in Changesets pre-release mode for the v13 RC, so the new workflow deliberately rejects every labeled PR; the failure text and docs also still point at the old v8 beta plan. More importantly, the npm trusted-publisher registration called out in the PR body is not provisioned, so the documented publish path cannot succeed end-to-end yet.

Please rebase onto current main, update the pre-mode/docs language for the current release process, provision the beta-release.yml trusted publisher (and decide/provision the cleanup token), then validate one labeled PR through publish and cleanup. Leaving this open for that follow-up rather than merging dormant, unverified release infrastructure.

[bokelley]

Closing this unmergeable release-infrastructure experiment for the current release train. The workflow was written for the old v8 beta process and its required npm trusted-publisher/cleanup credentials were never provisioned, so shipping it would add a path that cannot complete. The 3.2 prerelease should use the existing Changesets pre-mode pipeline; a per-PR snapshot workflow can return as a fresh PR after its publisher identity is provisioned and validated end to end.