Add workspace package onboarding

[temiroff]

What changed

• add a one-time first-workspace welcome dialog that opens the Packages tab
• persist acknowledgement in .blacknode/onboarding.json through the editor server instead of browser storage
• acknowledge onboarding only after an explicit user action
• add blacknode-training to the official package catalog
• use explicit .\start.bat commands throughout Windows documentation and launcher guidance
• document the workspace onboarding behavior
• add backend, editor-contract, catalog, and documentation regression tests

Why

A fresh Blacknode clone should guide users to official packages before package-backed robotics workflows. Browser storage made the experience inconsistent across clones and browser profiles, and an unpublished test clone could not exercise the new flow. Workspace-local persistence makes first-run behavior deterministic and keeps the acknowledgement out of Git.

Validation

• python -m pytest -q: 595 passed, 16 skipped, 45 subtests passed
• PYTHONPATH=python python -m unittest discover -s tests: 312 passed, 8 skipped
• npm run build: passed
• focused onboarding and package-index tests: passed
• git diff --check: passed