fix(copy): skip sync for files with unknown comment extensions instead of crashing

[EspenAlbert]

Summary

• Guard against ValueError in _handle_sync for files with extensions not in EXTENSION_COMMENT_MAP or FILENAME_COMMENT_MAP
• Files without a known comment config (e.g., .json, .cfg) caused a hard crash during sync because get_comment_config() raises on unknown extensions
• Files with unknown extensions are now skipped in sync mode with a warning log, since headers and sections require comment syntax to function

Details

_handle_sync calls sections.has_sections(), sections.wrap_in_synced_section(), and header.add_header(), all of which call get_comment_config(path) internally. For file extensions not in the known maps, this raises a ValueError and crashes the entire copy command.

The fix adds an early return at the top of _handle_sync using header.has_known_comment_prefix(dest_path). This is consistent with _find_files_with_config which already had the same guard for orphan cleanup.

SCAFFOLD and REPLACE sync modes are unaffected -- they don't need comment configs.