fix: update system prompt metadata label from "Memory blocks were last modified" to "System prompt last recompiled"
When git-based memory is enabled, there are no memory blocks, so the label
"Memory blocks were last modified" is inaccurate. Changed to
"System prompt last recompiled" which accurately reflects the timestamp meaning.
Fixes#9476🐾 Generated with [Letta Code](https://letta.com)
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Co-authored-by: Letta <noreply@letta.com>