feat(handoff): add explicit instructions to handoff nudge message

When a session starts via handoff, the nudge message now includes
clear instructions to check hook and mail. This prevents agent
confusion when SessionStart hooks haven't loaded CLAUDE.md yet.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/session/startup_test.go

@@ -54,6 +54,9 @@ func TestFormatStartupNudge(t *testing.T) {
 				"gastown/witness",
 				"<- self",
 				"handoff",
+				"Check your hook and mail", // handoff includes explicit instructions
+				"gt hook",
+				"gt mail inbox",
 			},
 		},
 		{