Make published plugin bundles reproducible

[nicosuave]

Follow-up to #53. Two problems with scripts/bundle-plugin.ts:

• Its output included esbuild's module path comments, which contain the builder's random temporary directory, so no two builds were the same bytes. The bundle is now minified, so the same source and checkout give identical output anywhere. That lets a deployment repo commit a bundle and check it against its source in CI.
• It loaded the plugin definition with a plain Bun import. A plugin kept in another repository couldn't resolve @sidequery/artifacts/plugins, and in a directory without node_modules Bun auto-installed the old npm release instead. The definition is now read from an esbuild bundle of the module, using the same alias as the shipped bundle.

A new test builds the fixture from another directory and expects identical bytes and no machine paths. The Miniflare runtime-plugin tests (6/0), the celld runtime-plugin test, and typecheck pass.