Add YARD documentation to source_prefix method

[Copilot]

The source_prefix method in LogEntryFormatters::BaseFormatter was missing YARD documentation while all other protected methods in the class had it.

Added documentation following the existing YARD style:

# Prefix emoji for the log entry based on its source.
# @param log_entry [LogEntry] The log entry to get the source prefix for.
# @return [String] The emoji prefix for the source (🐙 for github, ✍️ for manual, empty string otherwise).
def source_prefix(log_entry)
  # ...
end

This maintains consistency with the documentation patterns used in epic_prefix, metadata, and replace_people_handles.

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[github-actions]

Package	Line Rate	Complexity	Health
worklog	79%	0	✔
Summary	79% (743 / 944)	0	✔