Sort files section alphabetically in template.lock

[Copilot]

_write_lock was preserving the arbitrary insertion order of lock.files when serialising to template.lock, making diffs noisy and the output non-deterministic across syncs.

Changes

• _sync_helpers.py: existing_files is now built with sorted(...) and lock = dataclasses.replace(lock, files=existing_files) is unconditional — previously the replace only happened when files were missing from disk, so a no-missing-files sync left the list unsorted.
• test_sync.py: Tightened test_write_lock_preserves_all_existing_files to assert exact order rather than sorted(data["files"]). Added test_write_lock_files_sorted_alphabetically to cover the unsorted-input case directly.

# Before: order depended on input
files=["z.txt", "a.txt", "m.txt"]  →  written as ["z.txt", "a.txt", "m.txt"]

# After: always alphabetical
files=["z.txt", "a.txt", "m.txt"]  →  written as ["a.txt", "m.txt", "z.txt"]

Original prompt

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This section details on the original issue you should resolve

<issue_title>When writing template.lock</issue_title>
<issue_description>Write the file section in alphabetical order</issue_description>

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

• Fixes When writing template.lock #421

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