feat(npm): distribute the CLI via npm

[pjcdawkins]

Summary

Adds the build and publish tooling for distributing the Upsun CLI as
an npm package, so users can run npm install -g upsun or npx upsun.

Implements the optionalDependencies pattern used by esbuild, swc,
biome, turbo, and similar tools. A thin wrapper package (upsun)
declares four platform-specific packages as optionalDependencies;
npm uses each package's os and cpu fields to install only the
matching one. The wrapper's bin script resolves that package at
runtime and execs the embedded binary, forwarding argv, stdio, exit
code, and signals. No postinstall script and no runtime download.

Packages produced per release:

Package	Contents
upsun	wrapper
@upsun/cli-linux-x64	Linux amd64 binary
@upsun/cli-linux-arm64	Linux arm64 binary
@upsun/cli-darwin	macOS universal binary (x64 + arm64)
@upsun/cli-win32-x64	Windows amd64 binary

macOS uses the universal binary GoReleaser already builds, so a single
darwin package covers Apple Silicon and Intel.

publish.sh publishes the platform packages first, waits for them to
become queryable on the public registry, and only then publishes the
wrapper. The wait matters: npm publish returns success before the
package is visible to npm view, and any user running npx upsun in
that window gets a broken install in their npx cache that does not
self-heal.

Layout

• npm/wrapper: shim package with bin/upsun.js
• npm/platform-template: template for the per-platform packages
• npm/scripts/build.sh: assembles tarballs from GoReleaser archives
• npm/scripts/publish.sh: publishes platforms, waits, then wrapper
• Makefile targets: npm-pack, npm-publish, npm-clean

Out of scope

A GitHub Actions workflow that runs make npm-pack && make npm-publish
after the existing release job will follow in a separate PR. That step
also needs an NPM_TOKEN automation token added to the repo secrets.

Verification

The current snapshot version was published to the public registry from
this branch as a seed; npx upsun --version resolves and runs the real
binary end to end. Build, install, exec, argv passthrough, exit-code
propagation all confirmed.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds npm build/publish tooling to distribute the Upsun CLI via a wrapper package (upsun) plus platform-specific optionalDependency packages, enabling npm install -g upsun / npx upsun without postinstall downloads.

Changes:

• Add npm wrapper package (bin shim + package metadata) that resolves the correct platform package at runtime and execs the embedded binary.
• Add scripts to build npm tarballs from GoReleaser archives and publish them in a safe order (platforms first, then wrapper after propagation).
• Add templates/docs + Makefile targets and ignore npm/dist/ output.

Reviewed changes

Copilot reviewed 8 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
npm/wrapper/package.json.tmpl	Defines the upsun wrapper package and optionalDependencies on platform packages.
npm/wrapper/bin/upsun.js	Runtime resolver/launcher for the embedded platform binary.
npm/wrapper/README.md	Wrapper package README for npm consumers.
npm/scripts/build.sh	Builds wrapper + platform npm packages from GoReleaser artifacts.
npm/scripts/publish.sh	Publishes platform packages, waits for registry visibility, then publishes wrapper.
npm/platform-template/package.json.tmpl	Template for per-platform package metadata (os/cpu gated).
npm/platform-template/README.md.tmpl	Template README for per-platform packages.
npm/README.md	Repository docs for npm distribution workflow.
Makefile	Adds npm-pack, npm-publish, npm-clean targets.
.gitignore	Ignores npm/dist/ build outputs.

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