fix(startup): add actionable instructions to assigned beacon

Polecats were burning 48k+ tokens on exploratory work when spawned because
the startup beacon was informational-only. By the time the propulsion nudge
arrived 2 seconds later, the agent had already started exploring.

The handoff topic already had explicit instructions; this adds the same
pattern for assigned work: "Work is on your hook. Run gt hook now."

Fixes #319

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

internal/session/startup_test.go

@@ -25,6 +25,8 @@ func TestFormatStartupNudge(t *testing.T) {
 				"gastown/crew/gus",
 				"<- deacon",
 				"assigned:gt-abc12",
+				"Work is on your hook", // assigned includes actionable instructions
+				"gt hook",
 			},
 		},
 		{