refactor: split monolithic models.py into a models/ subpackage

[Copilot]

src/rhiza/models.py (~962 lines) contained four unrelated dataclasses, git subprocess helpers, YAML I/O, and path resolution logic in a single file, making it hard to navigate and unit-test in isolation.

Changes

New package structure

src/rhiza/models/
  __init__.py      # re-exports all public symbols — zero changes needed in callers
  _git_utils.py    # get_git_executable, _log_git_stderr_errors, _normalize_to_list
  bundle.py        # BundleDefinition, RhizaBundles
  template.py      # RhizaTemplate (~500 lines)
  lock.py          # TemplateLock

Backward compatibility

__init__.py re-exports the full public API, so all existing imports across commands/ continue to work unchanged:

# still works — no changes required in callers
from rhiza.models import RhizaTemplate, TemplateLock, get_git_executable

Test adjustments

• test_models.py, test_bundle_resolver.py, test_subprocess_utils.py — imports updated to reference specific submodules (e.g. from rhiza.models.bundle import RhizaBundles)
• test_commands/test_sync.py — subprocess.run mock path updated from rhiza.models.subprocess.run → rhiza.models.template.subprocess.run

Original prompt

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This section details on the original issue you should resolve

<issue_title>Refactor</issue_title>
<issue_description>1. 🗂️ Split the Monolithic models.py into Smaller Modules
File: src/rhiza/models.py (~962 lines)

This single file contains four unrelated dataclasses (BundleDefinition, RhizaBundles, RhizaTemplate, TemplateLock), git-subprocess helpers, YAML I/O, and path resolution logic. RhizaTemplate alone is ~500 lines.

Suggested split:

Code
src/rhiza/
models/
init.py # re-exports for backward compatibility
bundle.py # BundleDefinition, RhizaBundles
template.py # RhizaTemplate
lock.py # TemplateLock
_git_utils.py # get_git_executable, _log_git_stderr_errors
This improves navigability, reduces merge conflicts, and makes each class unit-testable in isolation.</issue_description>

<agent_instructions>also adjust the tests then</agent_instructions>

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

• Fixes Refactor #396

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