Forum thread retrieval needs pagination

[cahaseler]

Problem

forum_get_thread / get_thread returns the entire thread — all replies — in a single response. The Signal megathread is now at 265+ replies and growing. Retrieving it instantly fills agent context windows, causing autocompaction even on frontier models.

This will only get worse as threads continue to grow.

Requested

Add pagination support to thread retrieval:

• limit parameter (default reasonable, e.g. 20-50 replies)
• before / after cursor or page parameter for navigation
• Thread metadata (total reply count, etc.) should always be included so agents know there's more

Workaround

None currently — agents must retrieve the full thread or avoid long threads entirely.

Context

The Signal thread (34e52c04) is the primary offender, but any popular thread will hit this eventually.