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)
 		}

internal/cmd/sling.go

@@ -104,6 +104,7 @@ var (
 	slingCreate   bool   // --create: create polecat if it doesn't exist
 	slingForce    bool   // --force: force spawn even if polecat has unread mail
 	slingAccount  string // --account: Claude Code account handle to use
+	slingAgent    string // --agent: override runtime agent for this sling/spawn
 	slingNoConvoy bool   // --no-convoy: skip auto-convoy creation
 )
 
@@ -120,6 +121,7 @@ func init() {
 	slingCmd.Flags().BoolVar(&slingCreate, "create", false, "Create polecat if it doesn't exist")
 	slingCmd.Flags().BoolVar(&slingForce, "force", false, "Force spawn even if polecat has unread mail")
 	slingCmd.Flags().StringVar(&slingAccount, "account", "", "Claude Code account handle to use")
+	slingCmd.Flags().StringVar(&slingAgent, "agent", "", "Override agent/runtime for this sling (e.g., claude, gemini, codex, or custom alias)")
 	slingCmd.Flags().BoolVar(&slingNoConvoy, "no-convoy", false, "Skip auto-convoy creation for single-issue sling")
 
 	rootCmd.AddCommand(slingCmd)
@@ -243,6 +245,7 @@ func runSling(cmd *cobra.Command, args []string) error {
 					Account:  slingAccount,
 					Create:   slingCreate,
 					HookBead: beadID, // Set atomically at spawn time
+					Agent:    slingAgent,
 				}
 				spawnInfo, spawnErr := SpawnPolecatForSling(rigName, spawnOpts)
 				if spawnErr != nil {
@@ -849,6 +852,7 @@ func runSlingFormula(args []string) error {
 					Naked:   slingNaked,
 					Account: slingAccount,
 					Create:  slingCreate,
+					Agent:   slingAgent,
 				}
 				spawnInfo, spawnErr := SpawnPolecatForSling(rigName, spawnOpts)
 				if spawnErr != nil {
@@ -1101,7 +1105,6 @@ func agentIDToBeadID(agentID, townRoot string) string {
 	}
 }
 
-
 // IsDogTarget checks if target is a dog target pattern.
 // Returns the dog name (or empty for pool dispatch) and true if it's a dog target.
 // Patterns:
@@ -1395,6 +1398,7 @@ func runBatchSling(beadIDs []string, rigName string, townBeadsDir string) error
 			Account:  slingAccount,
 			Create:   slingCreate,
 			HookBead: beadID, // Set atomically at spawn time
+			Agent:    slingAgent,
 		}
 		spawnInfo, err := SpawnPolecatForSling(rigName, spawnOpts)
 		if err != nil {