Add registerMemoryPromptSection to the plugin API so the active memory
plugin provides its own system prompt builder callback. The built-in
memory-core plugin registers the existing Memory Recall section via
this new API.
When no memory plugin is active (or a plugin does not register a
builder), the memory section is simply omitted from the system prompt.
This lets third-party memory plugins inject their own guidance at the
same position in the prompt.
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