feat(memory): structured summarization with typed key facts

[bug-ops]

Problem

Summarization currently produces free-form text. Key facts and entities are not separately addressable for semantic search or retrieval.

Solution

Use chat_typed to produce structured summaries:

#[derive(Deserialize, JsonSchema)]
struct StructuredSummary {
    summary: String,
    key_facts: Vec<String>,
    entities: Vec<String>,
}

Store key_facts and entities as separate Qdrant points for better retrieval. The summary field replaces the current free-form output.

Acceptance criteria

• StructuredSummary struct with JsonSchema derive
• Updated summarize() in SemanticMemory to use chat_typed
• Individual key facts stored as Qdrant points
• Fallback to plain text summary if structured parsing fails
• Unit tests with MockProvider

Part of Epic #549