Feature request: section/chapter-scoped compaction

Problem

Today, /compact and autocompact operate over the entire conversation context as one unit. There is no way to compact just one part of a session.

After a noisy phase — a long exploration, a big file/log dump, a debugging detour that is now resolved — you often want to collapse that portion into a short summary while keeping everything before and after it fully intact. The only current lever is /compact [focus], which still compacts everything and only biases what is kept; it cannot target a region.

Proposed feature

Allow compaction to be scoped to a selected section of the session. Chapters (the existing transcript chapter / table-of-contents markers) are a natural unit: let users collapse a specific chapter into a summary on demand, leaving the rest of the context untouched.

Example

• /compact chapter:2 — summarize only chapter 2's messages, keep all other chapters verbatim
• Or a UI affordance on a chapter in the table of contents: "collapse this chapter"

Why it's useful

• Reclaim context budget precisely instead of blurring the whole session
• Preserve detail in the parts that still matter while discarding resolved noise
• More predictable than hoping a focus hint protects the right content

Related

Message pinning requests (#32874, #60992) target a similar need — surviving compaction — from the opposite direction (protect rather than collapse).