Epic: #1973

Extract crates/zeph-core/src/sanitizer/ (4,337 LOC) into standalone zeph-sanitizer crate.

Modules: ContentSanitizer, exfiltration, pii, quarantine, memory_validation, guardrail.

Acceptance Criteria:

• zeph-sanitizer compiles independently
• zeph-core re-exports public types for backward compat
• Feature flags (guardrail) move to new crate

Complexity: Medium | Effort: 4h