fix(server): retain Grok notification consumer for session

[patroza]

Root cause

The keyed provider-command lanes made command fibers intentionally short-lived. Grok's notification consumer was still attached with forkChild, so it was interrupted when the lane that created the session became idle. Completed Grok prompts then blocked forever in drainEvents, leaving T3 projections at running.

Fix

• attach the notification consumer to the existing Grok session scope
• add a regression test that starts the session from a short-lived fiber, lets that fiber exit, then verifies a prompt still settles

Tests

• vp check
• ELECTRON_SKIP_BINARY_DOWNLOAD=1 vp run -r --cache --log labeled typecheck
• vp test run apps/server/src/provider/Layers/GrokAdapter.test.ts (28 passed)