docs(sandbox): install Orca plugin by git URL

[pratikbin]

What

createos sandbox setup orca still printed the old Dev Paths install steps. The plugin now lives in its own repo, NodeOps-app/createos-orca-plugin, with orca-plugin.json at the clone root. Orca's git-URL installer accepts it.

• Replaced orcaPluginRepoURL and orcaPluginPkgPath with one pinned constant, orcaPluginGitURL.
• Rewrote the help text: five steps become four, and step 1 plus 2 are one paste.
• Deleted the closing paragraph that said git-URL install does not work.
• Updated the same claim in README.md.

The URL pins tag v0.1.0. An unpinned URL tracks the default branch and can change under an installed workspace.

Verified

go build ./..., go vet, and golangci-lint run ./cmd/sandbox/... all pass. Live output from the built binary:

Next, install the Orca plugin. Orca performs the install itself,
so it can show you what the plugin contributes before you approve it.

  1. Open Orca > Settings > Plugins > Install from git URL
  2. Paste https://github.com/NodeOps-app/createos-orca-plugin.git#v0.1.0
  3. Approve the "CreateOS Sandbox" VM recipe when Orca asks
  4. Turn on Settings > Cloud VM

Then pick "CreateOS Sandbox" when you create a workspace, under
Run on > Per-Workspace Environment.

Text only. No behavior changed.