Establish GitContext dataclass for injectable git context

[Copilot]

_get_diff was the only git-invoking function in _sync_helpers.py that resolved its own executable via get_git_executable() internally, making binary injection in tests impossible and breaking consistency with every other git-calling function in the module.

Changes

• New GitContext dataclass (rhiza.models._git_utils) — bundles executable: str and env: dict[str, str]; exposes a GitContext.default() classmethod that mirrors the setup previously scattered across callers
• Exported from rhiza.models — available as a first-class public API symbol
• _get_diff now accepts git_ctx: GitContext — no longer resolves the binary itself; consistent with all other helpers in the module
• _sync_diff propagates git_ctx: GitContext — threading the context down to _get_diff
• _merge_with_base — constructs a GitContext inline from its existing git_executable/git_env params; no signature change required
• sync.py — passes GitContext to _sync_diff

# Before — not injectable, inconsistent with peer functions
def _get_diff(repo0: Path, repo1: Path) -> str:
    git = get_git_executable()  # resolved internally
    ...

# After — explicit context, injectable in tests
def _get_diff(repo0: Path, repo1: Path, git_ctx: GitContext) -> str:
    ...subprocess.run([git_ctx.executable, ...], env=git_ctx.env, ...)

# Tests can now inject a custom binary without patching globals
ctx = GitContext(executable="/usr/bin/fake-git", env={})
diff = _get_diff(base, upstream, ctx)

Tests

• TestGitContext added to test_subprocess_utils.py covering direct construction, default env values, and default() classmethod behaviour
• All 16 _get_diff and _sync_diff call sites in test_sync.py updated to pass an explicit GitContext

Original prompt

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This section details on the original issue you should resolve

<issue_title>Establish a GitContext Dataclass</issue_title>
<issue_description>_get_diff was the only git-invoking function in _sync_helpers.py that resolved its own executable internally via get_git_executable(), making it impossible to inject a different binary in tests and breaking consistency with every other git-calling function in the module.
GitContext could help here</issue_description>

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

• Fixes Establish a GitContext Dataclass #414

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