feat(types): add custom type support to beads config (bd-649s)

Add types.custom config key mirroring the status.custom pattern:
- Add CustomTypeConfigKey constant and GetCustomTypes() to storage interface
- Add IssueType.IsValidWithCustom() method for validation
- Add ValidateWithCustom() to Issue for combined status/type validation
- Update all validation call sites to use GetCustomTypes()
- Rename parseCustomStatuses to parseCommaSeparated for reuse

This enables Gas Town to register custom types like agent/role/rig/convoy
without hardcoding them in beads core, supporting the type extraction epic.

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

internal/storage/memory/memory.go

@@ -1648,8 +1648,8 @@ func (m *MemoryStorage) GetCustomStatuses(ctx context.Context) ([]string, error)
 	return parseCustomStatuses(value), nil
 }
 
-// parseCustomStatuses splits a comma-separated string into a slice of trimmed status names.
-func parseCustomStatuses(value string) []string {
+// parseCommaSeparated splits a comma-separated string into a slice of trimmed values.
+func parseCommaSeparated(value string) []string {
 	if value == "" {
 		return nil
 	}
@@ -1664,6 +1664,23 @@ func parseCustomStatuses(value string) []string {
 	return result
 }
 
+// Alias for backwards compatibility in tests
+func parseCustomStatuses(value string) []string {
+	return parseCommaSeparated(value)
+}
+
+// GetCustomTypes retrieves the list of custom issue types from config.
+func (m *MemoryStorage) GetCustomTypes(ctx context.Context) ([]string, error) {
+	value, err := m.GetConfig(ctx, "types.custom")
+	if err != nil {
+		return nil, err
+	}
+	if value == "" {
+		return nil, nil
+	}
+	return parseCommaSeparated(value), nil
+}
+
 // Metadata
 func (m *MemoryStorage) SetMetadata(ctx context.Context, key, value string) error {
 	m.mu.Lock()