feat(namepool): auto-select theme per rig based on name hash

Each rig now gets a deterministic theme based on its name instead of
always defaulting to mad-max. Uses a prime multiplier hash (×31) for
good distribution across themes. Same rig name always gets the same
theme. Users can still override with `gt namepool set`.

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

internal/polecat/namepool.go

@@ -103,7 +103,7 @@ type NamePool struct {
 func NewNamePool(rigPath, rigName string) *NamePool {
 	return &NamePool{
 		RigName:      rigName,
-		Theme:        DefaultTheme,
+		Theme:        ThemeForRig(rigName),
 		InUse:        make(map[string]bool),
 		OverflowNext: DefaultPoolSize + 1,
 		MaxSize:      DefaultPoolSize,
@@ -352,6 +352,21 @@ func ListThemes() []string {
 	return themes
 }
 
+// ThemeForRig returns a deterministic theme for a rig based on its name.
+// This provides variety across rigs without requiring manual configuration.
+func ThemeForRig(rigName string) string {
+	themes := ListThemes()
+	if len(themes) == 0 {
+		return DefaultTheme
+	}
+	// Hash using prime multiplier for better distribution
+	var hash uint32
+	for _, b := range []byte(rigName) {
+		hash = hash*31 + uint32(b)
+	}
+	return themes[hash%uint32(len(themes))]
+}
+
 // GetThemeNames returns the names in a specific theme.
 func GetThemeNames(theme string) ([]string, error) {
 	if names, ok := BuiltinThemes[theme]; ok {