Feature proposal: export one channel's messages and threads to Markdown #180

Could slk support exporting one channel's messages and threaded replies for a date range? This would extend the usefulness of saving individual threads with S.

slk export \
 --workspace example-workspace \
 --channel project_alpha \
 --since 2026年04月01日 \
 --until 2026年07月01日 \
 --timezone America/New_York \
 --output ./project-alpha-export \
 --overlap 14

• Use existing workspace credentials to fetch one channel directly, without launching the TUI.
• Save each conversation, including standalone messages, to a Markdown file with an index.md linking them together.
• Treat --since as inclusive and --until as exclusive in the selected timezone.
• Let --overlap N add N calendar days before --since and after --until (default: zero).
• Include replies within the expanded interval even when their parent is older, retaining that parent as context.

Would you accept a PR for this feature? I'd like to confirm the scope and CLI approach before starting implementation.