Part of #1195 — Phase 2

Apply sanitization to memory search results (Qdrant/SQLite) to prevent poisoned memory attacks.

Crates: zeph-memory
Depends on: SEC-1.4

Tasks:

• SemanticMemory::search() results tagged ExternalUntrusted (memory could have been poisoned by earlier injected content)
• Apply ContentSanitizer to retrieved memory entries before returning to agent
• tracing::warn! if injection patterns detected in stored memory (indicates prior poisoning)
• Unit tests with mock memory entry containing injection payload

Files: crates/zeph-memory/src/orchestrator.rs