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/session/startup_test.go

@@ -86,6 +86,35 @@ func TestFormatStartupNudge(t *testing.T) {
 				"ready",
 			},
 		},
+		{
+			name: "start includes fallback instructions",
+			cfg: StartupNudgeConfig{
+				Recipient: "beads/crew/fang",
+				Sender:    "human",
+				Topic:     "start",
+			},
+			wantSub: []string{
+				"[GAS TOWN]",
+				"beads/crew/fang",
+				"<- human",
+				"start",
+				"gt prime", // fallback instruction for when SessionStart hook fails
+			},
+		},
+		{
+			name: "restart includes fallback instructions",
+			cfg: StartupNudgeConfig{
+				Recipient: "gastown/crew/george",
+				Sender:    "human",
+				Topic:     "restart",
+			},
+			wantSub: []string{
+				"[GAS TOWN]",
+				"gastown/crew/george",
+				"restart",
+				"gt prime", // fallback instruction for when SessionStart hook fails
+			},
+		},
 	}
 
 	for _, tt := range tests {