[Blueprints] Allow skipping plugin install failures

[adamziel]

What?

Adds installPlugin.options.onError with two modes:

Value	Behavior
throw	Default. Installation or activation failures abort the Blueprint, matching existing behavior.
skip-plugin	Logs a warning and lets the Blueprint continue when plugin installation or activation fails.

Also adds installPlugin.options.humanReadableName so skipped plugin warnings and progress text can use a useful display name when the source filename or URL is not readable.

Why?

Some Blueprints install optional plugins, especially compatibility-testing Blueprints that target nightly or beta WordPress/PHP combinations. In those cases, one incompatible plugin should be visible in logs without preventing the rest of the environment from booting.

This keeps the default behavior strict and only changes execution when the Blueprint explicitly opts in with skip-plugin.

Consumers

Consumer	Benefit
Blueprint v2 runner	Can map plugin-level onError: "skip-plugin" from the v2 spec to the existing installPlugin step.
Compatibility/testing Blueprints	Can continue setting up the site when an optional plugin fails against a target WP/PHP version.
CLI and browser Playground users	Get a warning for skipped plugins instead of a hard setup failure when the Blueprint author marked the plugin optional.
Existing v1 Blueprint consumers	No behavior change unless they opt in through installPlugin.options.onError.

Tests

• npm exec nx run playground-blueprints:build:blueprint-schema
• npm exec nx run playground-blueprints:test:vite -- --testFiles=packages/playground/blueprints/src/tests/steps/install-plugin.spec.ts
• npm exec nx run playground-blueprints:test:vite -- --testFiles=packages/playground/blueprints/src/tests/v1/compile.spec.ts
• npm exec nx run playground-blueprints:lint
• npm exec nx run playground-blueprints:typecheck
• git diff --check