Align Claude chat importer with segment output pattern
Rewrite ClaudeChatImporter to extract per-message timestamps, window
into 5-minute groups, and write segment files via write_segment —
matching the ChatGPT importer pattern from 2abe4619.
Extract _window_messages() from chatgpt.py into shared.py so both
importers share the same windowing logic.
Output path changes from YYYYMMDD/import.claude/imported.jsonl to
YYYYMMDD/import.claude/HHMMSS_300/imported_audio.jsonl. ImportResult
now populates segments for CLI stream marker and callosum event
handling.