### Overview
This commit introduces a broad set of improvements across the defaults
generator, credential creation subsystem, inventory creation workflow,
and InventoryManager core logic.
### Major Changes
- Support empty or config/main.yml in defaults generator and ensure that
applications with empty configs are still included in defaults_applications.
- Add '--snippet' and '--allow-empty-plain' modes to create/credentials.py
with non-destructive merging and correct plain-secret handling.
- Ensure empty strings for 'plain' credentials are never encrypted.
- Update InventoryManager to fully support allow_empty_plain and prevent
accidental overwriting or encrypting existing VaultScalar or dict values.
- Add full-size implementation of cli/create/inventory.py including
dynamic inventory building, role filtering, host_vars management, and
parallelised credential snippet generation.
- Fix schemas (Magento, Nextcloud, OAuth2-Proxy, keyboard-color, etc.) to
align with the new credential model and avoid test failures.
- Improve get_app_conf consistency by ensuring credentials.* paths are
always resolvable for applications even when config/main.yml is empty.
### Added Test Coverage
- Unit tests for defaults generator handling empty configs.
- Full test suite for create/inventory.py including merge logic and
vault-safe host_vars loading.
- Extensive tests for InventoryManager: plain-secret behavior,
vault handling, and recursion logic.
- Update or remove outdated tests referencing old schema behaviour.
### Context
This commit is associated with a refactoring and debugging session documented here:
https://chatgpt.com/share/692ec0e1-5018-800f-b568-d09a53e9d0ee
