Prevent absent files from being recorded in template.lock

[Copilot]

template.lock could list files under files: that don't exist in the target repo — e.g. when a 3-way merge conflict prevented a file from being written, or when a tracked file was manually deleted.

Changes

src/rhiza/commands/_sync_helpers.py

• _write_lock now filters lock.files to paths that actually exist under target before persisting. Missing paths are dropped with a warning; the original TemplateLock object is never mutated (dataclasses.replace used for the filtered copy).

# Before: materialized files from upstream snapshot written verbatim
lock.to_yaml(tmp_path)

# After: only paths present on disk survive into the lock
existing_files = [f for f in lock.files if (target / f).exists()]
lock = dataclasses.replace(lock, files=existing_files)
lock.to_yaml(tmp_path)

tests/test_commands/test_sync.py

• test_write_lock_yaml_format: creates the referenced file on disk (required now that absent files are filtered).
• Three new cases added to TestLockFile: partial existence, all missing, and all present.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Files can be in template.lock without being in the repo</issue_title>
<issue_description>Make this impossible</issue_description>

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

• Fixes Files can be in template.lock without being in the repo #372

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