[pi-extension] Memory Recall Quality Improvements

[codenamekt]

Overview

Improve the quality and utility of memory recall in the pi-extension.

Issues to Address

• Memory content truncated at 400 chars — add maxMemoryLength config (default 800), truncate at word boundary
• No way to exclude/silence specific memories — add hexus_exclude tool to suppress a memory from future recall
• Memory ordering is arbitrary — sort results by recency (created_at) as a tiebreaker when scores are close
• No recall observability — log hexus_recall hits/misses for debugging

Acceptance Criteria

• Memories display up to 800 chars by default (configurable)
• hexus_exclude tool marks memories as excluded, they stop appearing in recall
• Results with similar scores ordered by recency
• Recall stats logged (count, latency)