Add fallback instructions to start/restart topics in FormatStartupNudge()
so agents have actionable instructions even if SessionStart hook fails.
Previously, "start" and "restart" beacons only contained metadata like:
[GAS TOWN] beads/crew/fang <- human • 2025-01-12 • start
If the SessionStart hook failed to inject context via `gt prime`, agents
would sit idle at "No recent activity" screen with no instructions.
Now these topics include:
Run `gt prime` now for full context, then check your hook and mail.
Also warn instead of silently discarding settings provisioning errors in
crew_at.go.
Fixes: gt-uoc64
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
605eeec84e