Say what the vibe load test catches and what it misses

[saworbit]

Summary

Doc follow up to #956.

• DEVELOPMENT.md and the vibe README say CI loads every scenario, so a parse error fails the build, and that a call through an untyped subsystem still only shows in the sweep ([CI] Catch a vibe scenario calling a removed subsystem method #958).
• tests/test_level_root_shims.gd said two rules and listed three.

Checks

• run_local_checks.py --check passes
• -gselect=test_level_root_shims passes
• git diff --check is clean