Standardise YAML serialization behind a shared Protocol and generic helper

[Copilot]

TemplateLock, RhizaTemplate, and RhizaBundles each duplicated from_yaml/to_yaml with no shared contract, making generic utilities and type-safe dispatch impossible.

Changes

• src/rhiza/models/_base.py (new) — introduces two public symbols:

  • YamlSerializable — @runtime_checkable Protocol declaring from_yaml(Path) -> Self and to_yaml(Path) -> None; all three models satisfy it structurally with no inheritance changes required
  • load_model(cls, path) — generic typed helper preserving the concrete return type via TypeVar, with an explicit TypeError guard for non-conforming classes

• bundle.py — adds RhizaBundles.to_yaml so all three models fully satisfy YamlSerializable

• models/__init__.py — re-exports YamlSerializable and load_model

Usage

from rhiza.models._base import load_model, YamlSerializable
from rhiza.models.lock import TemplateLock
from rhiza.models.template import RhizaTemplate

# Generic load — return type is inferred correctly by type checkers
lock = load_model(TemplateLock, Path(".rhiza/template.lock"))   # TemplateLock
tmpl = load_model(RhizaTemplate, Path(".rhiza/template.yml"))  # RhizaTemplate

# Runtime protocol check
assert isinstance(lock, YamlSerializable)

Original prompt

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This section details on the original issue you should resolve

<issue_title>yaml</issue_title>
<issue_description>10. 📝 Standardise to_yaml / from_yaml behind a common base class or Protocol

Files: src/rhiza/models/lock.py, src/rhiza/models/template.py, src/rhiza/models/bundle.py

All three models (TemplateLock, RhizaTemplate, RhizaBundles) implement from_yaml / to_yaml independently with no shared interface. Introduce a YamlSerializable Protocol (or abstract base YamlModel) in src/rhiza/models/_base.py to enforce the contract, improve type-checking, and allow generic utilities (e.g., a load_model(cls, path) helper).</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes yaml #404

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