feat: add --json output to conversations list (#153)

conversations list was the one read command without --json, while the README, the scripting guide and docs/development/setup.md all promised it. Emits {conversation_count, conversations, users, next_cursor} with the explicit field set agreed on the issue: SlackChannel id/name/user, the type and status flags, num_members and flattened topic/purpose, plus the conversations read user shape for resolved DM partners.

Slack returns next_cursor as an empty string rather than omitting it, so the JSON path normalises it to null. The branch returns straight after writeJson(), so neither the channel list nor the pagination hint can reach stdout.

Closes #141