spawn: Add work instruction + export SessionName

- Export session.Manager.SessionName for spawn.go access
- Add --address alias for --identity in mail inbox/check
- Send explicit work instruction to polecat after spawn
- Add CapturePaneLines and WaitForClaudeReady helpers (unused for now)
- Proper solution filed as gt-hb0 (needs Witness/Deacon AI monitoring)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/session/manager_test.go

@@ -16,7 +16,7 @@ func TestSessionName(t *testing.T) {
 	}
 	m := NewManager(tmux.NewTmux(), r)
 
-	name := m.sessionName("Toast")
+	name := m.SessionName("Toast")
 	if name != "gt-gastown-Toast" {
 		t.Errorf("sessionName = %q, want gt-gastown-Toast", name)
 	}