Migrate Taskboard and Usage Tracker to bb 0.38

[MateoCerquetella]

What changed

• migrate Taskboard and Usage Tracker from vendored SDK declarations to the published @get-bb/plugin-sdk@0.4.6 package
• raise both plugin compatibility floors to BB 0.38 and build with bb-app@0.38.0
• add .bb/plugins.json so BB 0.38 can select either plugin from this monorepo
• bump the next package versions to Taskboard 0.1.3 and Usage Tracker 0.1.2
• restore Usage Tracker's already-published 0.1.1 provider visibility settings and test to main, preventing a regression in 0.1.2
• update source-install docs, component registry pin, CI cleanliness check, and Node minimum

User impact

The next releases use BB's public SDK package and can be installed from the repository with --plugin taskboard or --plugin usage-tracker. Existing npm installs remain unchanged until those releases are published.

Checks

• npm run check
  • Taskboard: typecheck, 10 tests, BB build, artifact metadata verification
  • Usage Tracker: typecheck, 13 tests, BB build
• both server and app artifacts report BB 0.38.0 / plugin SDK 0.4.6
• .bb/plugins.json validates against the official collection schema
• package dry-runs contain both built artifacts and source entries, with no vendored SDK declarations