fix(prime): update sling prompts to use gt prime --hook

Polecats weren't getting full role context (including IDLE POLECAT
HERESY guidance) because start prompts said `gt hook` instead of
`gt prime --hook`. The SessionStart hook already runs `gt prime --hook`,
but manual nudge messages were inconsistent.

Changed:
- sling_helpers.go: nudge message now says `gt prime --hook`
- startup.go: assigned beacon now says `gt prime --hook`
- startup_test.go: updated test expectation

Fixes: hq-iqyu9

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

internal/session/startup_test.go

@@ -7,10 +7,10 @@ import (
 
 func TestFormatStartupBeacon(t *testing.T) {
 	tests := []struct {
-		name     string
-		cfg      BeaconConfig
-		wantSub  []string // substrings that must appear
-		wantNot  []string // substrings that must NOT appear
+		name    string
+		cfg     BeaconConfig
+		wantSub []string // substrings that must appear
+		wantNot []string // substrings that must NOT appear
 	}{
 		{
 			name: "assigned with mol-id",
@@ -26,7 +26,7 @@ func TestFormatStartupBeacon(t *testing.T) {
 				"<- deacon",
 				"assigned:gt-abc12",
 				"Work is on your hook", // assigned includes actionable instructions
-				"gt hook",
+				"gt prime --hook",      // full context including IDLE POLECAT HERESY
 			},
 		},
 		{