feat(core): pass compaction guidance to hooks and native compaction - #56679

Stacked on #56208.

Problem

Compaction guidance only reaches the summary prompt. Native provider compaction ignores it, and compaction hooks never see it.

Change

 compaction hook event
+  guidance?: string            // read-only

 native compaction request
   …window, fitted to the budget…
+  user: "Additional guidance for this summary:\n<guidance>"
   (trigger item or /responses/compact)

 endpoint replacement
-  stored as returned
+  stored without the guidance message

• The guidance message is added after the window is fitted, so it never displaces real history.
• It is removed from the provider's replacement window, so it is never stored or replayed.

Verification

• bun run check
• packages/core: native compaction tests (trigger and endpoint), runner test for the hook