feat(memory): pluggable system prompt section for memory plugins (#40126)

Merged via squash.

Prepared head SHA: 5228d1937f
Co-authored-by: jarimustonen <1272053+jarimustonen@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

src/plugin-sdk/memory-core.ts

@@ -2,4 +2,5 @@
 // Keep this list additive and scoped to symbols used under extensions/memory-core.
 
 export { emptyPluginConfigSchema } from "../plugins/config-schema.js";
+export type { MemoryPromptSectionBuilder } from "../memory/prompt-section.js";
 export type { OpenClawPluginApi } from "../plugins/types.js";