Refactor: migrate template helpers from _sync_helpers to RhizaTemplate private methods

[Copilot]

_sync_helpers.py contained 8 template-specific functions that operated directly on RhizaTemplate data, creating unnecessary coupling and module bloat. These have been moved to the class as private methods/static methods, eliminating deferred imports from models.py → _sync_helpers.

models.py — RhizaTemplate additions

• @staticmethod _expand_paths, _excluded_set, _prepare_snapshot — snapshot pipeline helpers
• @staticmethod _update_sparse_checkout, _get_head_sha — pure git utilities
• _clone_template_repository(self, ...), _clone_at_sha(self, ...) — instance methods using self.git_url
• from_project(cls, target, branch) — public classmethod replacing the standalone _validate_and_load_template; calls from_yaml internally with full validation
• clone() / snapshot() now call self._xxx() — no more deferred _sync_helpers imports

_sync_helpers.py — signature changes

_merge_with_base and _sync_merge now accept template: RhizaTemplate instead of separate git_url + include_paths params:

# Before
_sync_merge(..., include_paths=["Makefile"], excludes=set(), git_url="https://...", ...)

# After
_sync_merge(..., template=RhizaTemplate(template_repository="owner/repo", include=["Makefile"]), excludes=set(), ...)

sync.py

Uses RhizaTemplate.from_project(target, branch) (public API) and passes template directly to _sync_merge.

Tests

• Removed test classes that tested migrated private methods directly (TestExpandPaths, TestExcludedSet, TestPrepareSnapshot, TestGetHeadSha, TestCloneAtSha, TestCloneAtShaErrorPaths, TestCloneTemplateRepository, TestUpdateSparseCheckout, TestValidateAndLoadTemplate)
• Updated all @patch targets from rhiza.commands._sync_helpers.* → rhiza.models.RhizaTemplate.*
• Updated _merge_with_base / _sync_merge call sites to pass RhizaTemplate instances
• populate_base side-effect signatures drop the git_url positional arg (now self.git_url)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Refactor _sync_helpers: Move template-related helpers to RhizaTemplate as private attributes/methods</issue_title>
<issue_description>## Motivation

The module src/rhiza/commands/_sync_helpers.py currently contains many functions that closely operate on template data and would be better organized as private attributes/methods of the RhizaTemplate class (in rhiza/models.py). Migrating them improves encapsulation, reduces module bloat, and brings template logic together with its relevant data.

Candidates for Migration

Suggested functions to move to RhizaTemplate (with a _ prefix for class-private methods):

• _clone_template_repository(...) — Template repo cloning (already uses git_url, template_branch, include)
• _update_sparse_checkout(...) — Sparse-checkout update; relates to include paths
• _clone_at_sha(...) — Clone at a specific commit SHA for template
• _get_head_sha(...) — Get HEAD SHA of a cloned template repo
• _prepare_snapshot(...) — Build a snapshot from a clone using include/exclude
• _expand_paths(...) — Expand include paths relative to clone root
• _excluded_set(...) — Build exclude set from RhizaTemplate data
• _validate_and_load_template(...) — Loads and validates from template.yml, suitable for a factory/classmethod

Notes

• Many of these helpers use the same arguments/data as RhizaTemplate.clone() and RhizaTemplate.snapshot() (which are already class methods).
• Functions related to general-purpose git operations (diff application, orphan file cleanup) should remain as standalone helpers or move to a dedicated _git_helpers module.
• Refactoring should preserve testability (existing tests use direct imports).

Linked Areas

• RhizaTemplate class in src/rhiza/models.py
• Existing methods: clone(), snapshot()

Acceptance Criteria

• Template-related helpers migrated as private methods/attributes of RhizaTemplate
• Refactored helpers are invoked via self instead of standalone calls
• Module-level function bloat in _sync_helpers.py is reduced
• Tests updated to import methods as needed

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Discussion welcome on boundaries (what should stay as a function vs. a method) and any migration risks.
</issue_description>

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

• Fixes Refactor _sync_helpers: Move template-related helpers to RhizaTemplate as private attributes/methods #382

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