feat(apps): add publish option to upload without auto-publishing

[setnilson]

What

Adds a `publish` option to the apps plugin so high-code apps can be uploaded as drafts without immediately going live.

Previously, `npm run upload` always called `release/live` after a successful upload — there was no way to separate the two steps.

How

• New `publish?: boolean` option in `AppsOptions` (defaults to `true`, preserving existing behavior)
• `DD_APPS_PUBLISH=false` / `DATADOG_APPS_PUBLISH=false` env var as an alternative
• Explicit config takes precedence over the env var
• When `publish: false`, skips the `PUT .../release/live` call and logs a draft message instead

The `upload-no-publish` npm script that uses this is scaffolded automatically via `@datadog/create-apps` (separate web-ui PR). For existing projects, users can add it manually:

"upload-no-publish": "DD_APPS_UPLOAD_ASSETS=1 DD_APPS_PUBLISH=false vite build"

Changes

• `packages/core/src/helpers/env.ts` — registers `APPS_PUBLISH` env var
• `packages/plugins/apps/src/types.ts` — adds `publish` to `AppsOptions` and `AppsOptionsWithDefaults`
• `packages/plugins/apps/src/validate.ts` — resolves default from env var
• `packages/plugins/apps/src/upload.ts` — gates `release/live` call on `context.publish`
• `packages/plugins/apps/src/vite/handle-upload.ts` — passes `options.publish` through
• `packages/plugins/apps/README.md` — documents `apps.publish` and the `upload-no-publish` script pattern
• Tests updated throughout (235 passing)