Autosave assistant responses to semantic memory

[bug-ops]

Parent: #740 (P2)

Problem

Only user messages are stored via remember(). Agent-generated knowledge (answers, decisions, reasoning) does not accumulate in the long-term semantic store.

Solution

• After each assistant response, embed and store in semantic memory (above configurable min length threshold, e.g. 20 chars)
• Tag with role = "assistant" in Qdrant payload for filtering
• Config: memory.autosave_assistant (bool, default false)

Affected crates

• zeph-core

Acceptance criteria

• Assistant responses stored in semantic memory when enabled
• Retrievable via recall() with role filter
• Configurable min length threshold