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.go

@@ -152,6 +152,7 @@ type Storage interface {
 	GetAllConfig(ctx context.Context) (map[string]string, error)
 	DeleteConfig(ctx context.Context, key string) error
 	GetCustomStatuses(ctx context.Context) ([]string, error) // Custom status states from status.custom config
+	GetCustomTypes(ctx context.Context) ([]string, error)    // Custom issue types from types.custom config
 
 	// Metadata (for internal state like import hashes)
 	SetMetadata(ctx context.Context, key, value string) error