Block sync when git working tree is dirty

[Copilot]

rhiza sync could run against a repository with uncommitted changes, making it impossible to distinguish between local edits and sync-applied changes, and risking unrecoverable work.

Changes

• _sync_helpers.py — adds _assert_git_status_clean(): runs git status --porcelain on the target; raises RuntimeError with the list of dirty files if output is non-empty
• sync.py — calls _assert_git_status_clean() as the first operation in sync(), before branch handling or any template work
• test_sync.py — adds TestAssertGitStatusClean covering: clean tree (no raise), untracked file, and staged-but-uncommitted file

Behaviour

$ rhiza sync
[ERROR] Working tree is not clean. Please commit or stash your changes before syncing.
[ERROR] Uncommitted changes:
[ERROR]   ?? some-untracked-file.txt
[ERROR]  M modified-file.py

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make it impossible to do perform sync if git status is not clean</issue_title>
<issue_description></issue_description>

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

• Fixes Make it impossible to do perform sync if git status is not clean #370

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