fix(env): provision plugin vendor/ on env up to avoid activation fatal

[jason10lee]

All Submissions:

• Have you followed the Newspack Contributing guidelines?
• Does your code follow the WordPress coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

A fresh isolated environment (n env up) links the monorepo plugins but never installs their Composer vendor/ — so activating a plugin that requires its autoloader (e.g. newspack-plugin, newspack-popups) fatals on a missing vendor/autoload.php. This adds a bin/ensure-vendor.sh step at env-up that runs a runtime-only (--no-dev) per-plugin Composer install — all plugin activation needs, without dev dependencies or the slow JS build — so a fresh env is never vendorless. (Dev dependencies for running a plugin's tests still come from n build / n ci-build.)

• Idempotent. Skips any plugin whose vendor/autoload.php is already present, so warm starts stay instant.
• Non-fatal. If a Composer install fails, it warns actionably and leaves the env usable rather than tearing down an otherwise-working environment.

Only monorepo plugins are provisioned; standalone repos/ checkouts manage their own dependencies.

How to test the changes in this Pull Request:

1. n env up <name> on a fresh environment; confirm the [ensure-vendor] step installs Composer deps for the monorepo plugins.
2. Activate a plugin that requires its autoloader (e.g. newspack-plugin) — it activates without a fatal about a missing vendor/autoload.php.
3. Re-run n env up <name> — confirm ensure-vendor skips already-provisioned plugins (no redundant installs).
4. Force a Composer failure for one plugin — confirm env-up reports the failure (including how many other plugins were provisioned) but still completes and the env stays usable.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable? — no bin/ test harness in this repo.
• Have you successfully run tests with your changes locally? — verified in an isolated environment: ensure-vendor ran at env-up across the monorepo plugins, with error propagation + graceful degradation confirmed. No CI coverage for bin/ here, so verification is manual.

[Copilot]

Copilot wasn't able to review any files in this pull request.