refactor(cmd): extract common manager creation boilerplate

Add getRig() helper in rig_helpers.go that encapsulates the common
boilerplate for finding town root, loading rigs config, and retrieving
a rig. This reduces duplication across 5 get*Manager functions:
- getPolecatManager
- getSessionManager
- getCrewManager
- getRefineryManager
- getWitnessManager

Closes gt-7sqi.

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

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

internal/cmd/rig_helpers.go

@@ -0,0 +1,36 @@
+package cmd
+
+import (
+	"fmt"
+	"path/filepath"
+
+	"github.com/steveyegge/gastown/internal/config"
+	"github.com/steveyegge/gastown/internal/git"
+	"github.com/steveyegge/gastown/internal/rig"
+	"github.com/steveyegge/gastown/internal/workspace"
+)
+
+// getRig finds the town root and retrieves the specified rig.
+// This is the common boilerplate extracted from get*Manager functions.
+// Returns the town root path and rig instance.
+func getRig(rigName string) (string, *rig.Rig, error) {
+	townRoot, err := workspace.FindFromCwdOrError()
+	if err != nil {
+		return "", nil, fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+
+	rigsConfigPath := filepath.Join(townRoot, "mayor", "rigs.json")
+	rigsConfig, err := config.LoadRigsConfig(rigsConfigPath)
+	if err != nil {
+		rigsConfig = &config.RigsConfig{Rigs: make(map[string]config.RigEntry)}
+	}
+
+	g := git.NewGit(townRoot)
+	rigMgr := rig.NewManager(townRoot, rigsConfig, g)
+	r, err := rigMgr.GetRig(rigName)
+	if err != nil {
+		return "", nil, fmt.Errorf("rig '%s' not found", rigName)
+	}
+
+	return townRoot, r, nil
+}