Problem

slck messages thread <channel> <thread-ts> always returns the full thread (up to --limit). When polling a thread for new messages, the caller has to fetch all messages every cycle and filter client-side.

Proposal

Add a --since <timestamp> flag that only returns messages with ts greater than the given value.

slck messages thread C12345 1700000000.000000 --since 1700000050.000000 -o json

This maps to Slack's conversations.replies API parameter oldest.

Use case

Polling a thread for new messages at a regular interval (e.g. every 2s). Currently fetches up to 50 messages per poll and deduplicates client-side. With --since, the API call itself returns only new messages — less data, less filtering.

Not urgent — current approach works fine for typical thread sizes. Nice-to-have for efficiency.