Revert serialised plugin installs once adapt-cli race condition is fixed

[taylortom]

Context

In #80 we switched installPlugins from Promise.all (concurrent) to a sequential for...of loop as a workaround for a race condition in adapt-cli where multiple concurrent CLI commands read/write adapt.json simultaneously.

The underlying adapt-cli bugs are tracked in adaptlearning/adapt-cli#232:

• readValidateJSON silently swallows errors (returns undefined instead of re-throwing)
• getManifestDependencies doesn't handle undefined manifest
• project.add() has an unsafe read-modify-write cycle on adapt.json

Action

Once adaptlearning/adapt-cli#232 is resolved:

1. Revert the for...of loop back to Promise.all in installPlugins to restore concurrent plugin installation performance
2. Verify course imports with multiple plugins work correctly with concurrent installs

Why revert

Sequential installation is slower — each plugin waits for the previous one to complete. Concurrent installation is safe once the CLI properly handles concurrent file access.