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.go

@@ -62,6 +62,18 @@ func FormatStartupNudge(cfg StartupNudgeConfig) string {
 	}
 
 	// Build the beacon: [GAS TOWN] recipient <- sender • timestamp • topic
-	return fmt.Sprintf("[GAS TOWN] %s <- %s • %s • %s",
+	beacon := fmt.Sprintf("[GAS TOWN] %s <- %s • %s • %s",
 		cfg.Recipient, cfg.Sender, timestamp, topic)
+
+	// For handoff, add explicit instructions so the agent knows what to do
+	// even if hooks haven't loaded CLAUDE.md yet
+	if cfg.Topic == "handoff" {
+		beacon += "\n\nCheck your hook and mail, then act on the hook if present:\n" +
+			"1. `gt hook` - shows hooked work (if any)\n" +
+			"2. `gt mail inbox` - check for messages\n" +
+			"3. If work is hooked → execute it immediately\n" +
+			"4. If nothing hooked → wait for instructions"
+	}
+
+	return beacon
 }

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",
 			},
 		},
 		{