feat(sanitize): implement empty-block and payload preflight sanitizer

[sonwr]

Goal

Implement first production sanitizer for agent payload safety.

Scope

• Remove empty/whitespace-only text blocks
• Normalize message content arrays
• Add provider-aware preflight guard hooks

Acceptance Criteria

• Unit tests for empty-block stripping and normalization
• No behavior change for valid payloads
• Public API documented in package README