ci: add pkg-pr-new continuous release workflow

Split out of #2, which originally ported all three workflows from solidjs/solid-image.

The cr.yml workflow (pkg-pr-new preview releases) was dropped from that PR because it fails without the GitHub App installed:

Check failed (404): {"url":"/check","statusCode":404,"message":"There is no workflow defined for 2AnfDFy2wC"}

pkg-pr-new publish uploads the tarball, then asks the service to verify the calling workflow run via its GitHub App installation. With no installation on this repo there's nothing to attribute the run to, so the check 404s. solid-image works because the app is already installed there.

To resolve

1. Install https://github.com/apps/pkg-pr-new on solidjs/filesystem-routing (needs org owner / repo admin).
2. Restore .github/workflows/cr.yml — it can be copied verbatim from solid-image, or recovered from commit a9c9f11 on the ci/github-workflows branch.

No changes to the workflow itself are needed; it was correct as written.

🤖 Generated with Claude Code

First confirm that the pkg-pr-new GitHub App is installed on solidjs/filesystem-routing. Then inspect .github/workflows/cr.yml in commit a9c9f11 on the ci/github-workflows branch and restore it; done means the workflow runs and pkg-pr-new can create preview releases without the 404 check failure.