End-to-end example workspace bootstrap from fresh checkout

[tbitcs]

The current bootstrap e2e test uses a lightweight manifest stub. A full end-to-end test should exercise the real example workspace flow:

1. Copy example/workspace/ to a clean directory
2. Run scripts/bootstrap.sh (or .cmd)
3. Verify west update fetches Zephyr and west-env
4. Verify west env doctor passes
5. Build zephyr/samples/hello_world for native_sim

This is expensive (~10 min for Zephyr fetch) so it may be better as a scheduled nightly or manual workflow rather than on every push.

Acceptance criteria:

• A CI workflow (manual or scheduled) that runs the full bootstrap
• The workflow validates doctor + build succeed after bootstrap