fix(session): add fallback instructions to start/restart beacons

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>

internal/cmd/crew_at.go

@@ -103,7 +103,10 @@ func runCrewAt(cmd *cobra.Command, args []string) error {
 	}
 
 	runtimeConfig := config.LoadRuntimeConfig(r.Path)
-	_ = runtime.EnsureSettingsForRole(worker.ClonePath, "crew", runtimeConfig)
+	if err := runtime.EnsureSettingsForRole(worker.ClonePath, "crew", runtimeConfig); err != nil {
+		// Non-fatal but log warning - missing settings can cause agents to start without hooks
+		style.PrintWarning("could not ensure settings for %s: %v", name, err)
+	}
 
 	// Check if session exists
 	t := tmux.NewTmux()