feat: add --agent override for sling

internal/cmd/polecat_spawn.go

@@ -39,6 +39,7 @@ type SlingSpawnOptions struct {
 	Account  string // Claude Code account handle to use
 	Create   bool   // Create polecat if it doesn't exist (currently always true for sling)
 	HookBead string // Bead ID to set as hook_bead at spawn time (atomic assignment)
+	Agent    string // Agent override for this spawn (e.g., "gemini", "codex", "claude-haiku")
 }
 
 // SpawnPolecatForSling creates a fresh polecat and optionally starts its session.
@@ -122,8 +123,11 @@ func SpawnPolecatForSling(rigName string, opts SlingSpawnOptions) (*SpawnedPolec
 		fmt.Printf("Polecat created. Agent must be started manually.\n\n")
 		fmt.Printf("To start the agent:\n")
 		fmt.Printf("  cd %s\n", polecatObj.ClonePath)
-		// Use rig's configured agent command
-		agentCmd := config.ResolveAgentConfig(townRoot, r.Path).BuildCommand()
+		// Use rig's configured agent command, unless overridden.
+		agentCmd, err := config.GetRuntimeCommandWithAgentOverride(r.Path, opts.Agent)
+		if err != nil {
+			return nil, err
+		}
 		fmt.Printf("  %s\n\n", agentCmd)
 		fmt.Printf("Agent will discover work via gt prime on startup.\n")
 
@@ -157,6 +161,13 @@ func SpawnPolecatForSling(rigName string, opts SlingSpawnOptions) (*SpawnedPolec
 		startOpts := session.StartOptions{
 			ClaudeConfigDir: claudeConfigDir,
 		}
+		if opts.Agent != "" {
+			cmd, err := config.BuildPolecatStartupCommandWithAgentOverride(rigName, polecatName, r.Path, "", opts.Agent)
+			if err != nil {
+				return nil, err
+			}
+			startOpts.Command = cmd
+		}
 		if err := sessMgr.Start(polecatName, startOpts); err != nil {
 			return nil, fmt.Errorf("starting session: %w", err)
 		}