fix(polecat): pass agent override to startup command builder

When sling dispatches a polecat with --agent override (e.g., gemini),
the override was used to resolve the runtime config for fallback info
but NOT passed to BuildStartupCommandFromConfig. The command builder
re-resolved the agent from role_agents (defaulting to claude-opus),
so the actual tmux command launched claude instead of gemini.

Pass opts.Agent through so the correct agent binary and flags
(including -i for Gemini prompt injection) are used.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

internal/polecat/session_manager.go

@@ -299,7 +299,7 @@ func (m *SessionManager) Start(polecat string, opts SessionStartOptions) error {
 			Issue:       opts.Issue,
 			Topic:       "assigned",
 			SessionName: sessionID,
-		}, m.rig.Path, beacon, "")
+		}, m.rig.Path, beacon, opts.Agent)
 		if err != nil {
 			return fmt.Errorf("building startup command: %w", err)
 		}