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/storage_test.go

@@ -170,6 +170,9 @@ func (m *mockStorage) DeleteConfig(ctx context.Context, key string) error {
 func (m *mockStorage) GetCustomStatuses(ctx context.Context) ([]string, error) {
 	return nil, nil
 }
+func (m *mockStorage) GetCustomTypes(ctx context.Context) ([]string, error) {
+	return nil, nil
+}
 func (m *mockStorage) SetMetadata(ctx context.Context, key, value string) error {
 	return nil
 }
@@ -358,6 +361,7 @@ func TestInterfaceDocumentation(t *testing.T) {
 		_ = s.GetAllConfig
 		_ = s.DeleteConfig
 		_ = s.GetCustomStatuses
+		_ = s.GetCustomTypes
 
 		// Verify metadata operations
 		_ = s.SetMetadata