v4: Add custom exception hierarchy

[jtdub]

Description

Currently only DuplicateChildError exists as a custom exception. All other errors use generic ValueError or TypeError, making it difficult for users to catch specific failure modes.

Proposed Change

Add a proper exception hierarchy:

HierConfigError (base)
├── DriverNotFoundError       (replaces ValueError in get_hconfig_driver)
├── InvalidConfigError        (for malformed config text)
├── DuplicateChildError       (already exists, reparent under HierConfigError)
└── IncompatibleDriverError   (for WorkflowRemediation with mismatched drivers)

Breaking Change

Yes — DuplicateChildError moves under HierConfigError, and code catching ValueError from get_hconfig_driver() would need updating.