fix: improve local bootstrap for contributors

[hormers]

Summary

• make sync-remote reuse GitHub SSH remotes when the local repository is configured for SSH
• add a --clone-only path so just init only bootstraps the pinned upstream source trees needed for local development
• document that just init bootstraps pinned rolldown and vite sources
• wrap bootstrap build commands on macOS with an HTTP/1.1 curl shim to avoid flaky GitHub release downloads during local setup

Why

Local contributor setup could fail even when GitHub SSH access worked correctly.

Also, just init was running logic intended for full upstream dependency sync, which is heavier than the normal contributor bootstrap flow.

On macOS, local bootstrap could also fail when transitive build scripts downloaded GitHub release assets through curl and hit transport issues.

Scope

This PR includes:

• cross-platform bootstrap flow changes for just init / sync-remote
• documentation updates for contributor setup
• a macOS-only workaround in with-curl-http1.ts

The HTTP/1.1 curl shim is only applied on macOS. Other platforms keep their existing behavior.

Validation

Validated on macOS with:

• just build
• pnpm bootstrap-cli
• vp --version
• pnpm test

I did not validate the bootstrap flow on Windows in this PR.

[netlify]

✅ Deploy Preview for viteplus-preview canceled.

Name	Link
🔨 Latest commit	b96d888
🔍 Latest deploy log	https://app.netlify.com/projects/viteplus-preview/deploys/69bfa6cae27e5300086b2ebe

[chatgpt-codex-connector]

To use Codex here, create a Codex account and connect to github.