Part of #1195 — Phase 1

Define the trust boundary model used across all crates.

Crates: zeph-core

Tasks:

• TrustLevel enum: Trusted, LocalUntrusted, ExternalUntrusted
• ContentSource struct: source kind (tool, web, mcp, a2a, memory, file), origin URL/name, timestamp
• SanitizedContent struct: original content, trust level, source, detected flags, wrapped output
• Derive schemars::JsonSchema on public types
• Unit tests for type construction and serialization

Files: crates/zeph-core/src/sanitizer.rs (new), crates/zeph-core/src/lib.rs