Replace barrel imports in hub/__init__.py with dynamic registry-based resolution

[debu-sinha]

Context

Discussion #1412 and PR #1394 (shipped in v0.9.1) introduced the project-level
.guardrails/hub_registry.json registry. Currently guardrails hub install dual-writes:
it appends an import line to hub/__init__.py in site-packages AND writes to the JSON
registry.

The __init__.py mutation approach is fragile: it breaks on virtualenv changes, pip
upgrades, and concurrent installs. The registry was built to replace it.

@CalebCourier noted on Discussion #1412 (Mar 2): "If you have the bandwidth to open a
PR for Phase 2 it would be much appreciated."

Proposed Changes

1. Replace guardrails/hub/__init__.py barrel imports with PEP 562 __getattr__ that
   reads from hub_registry.json, caches the export map, and resolves validators lazily
2. Remove the add_to_hub_inits() call from the install path (registry becomes sole
   source of truth)
3. Add unregister_validator() for clean removal on uninstall
4. Rewrite guardrails hub list to read from registry instead of regex-parsing
   __init__.py

Backward Compatibility

• from guardrails.hub import DetectPII works identically via __getattr__
• Existing registry entries from v0.9.1 installs are used as-is
• add_to_hub_inits() and remove_from_hub_inits() stay in the codebase but are no
  longer called
• No migration command needed: reinstalling validators auto-populates the registry

Related

• Discussion Guardrails Hub CLI: uv support + other qol improvements #1412
• PR docs: Add MLflow integration guide #1394 (Phase 1)
• PR Add uv package manager support and project-level validator registry #1416 (Phase 1 follow-up)