Extract shared blake3 content hashing utility

[bug-ops]

Context

Seven crates use blake3 with slightly different patterns for content hashing.

Solution

Add a content_hash(data: &[u8]) -> String function to zeph-memory (or shared location) and reuse across all callers.

Acceptance Criteria

• Single content_hash utility used by all crates
• blake3 dependency consolidated

Epic: #832 | Effort: S | Crate: zeph-memory
Critical path: independent, parallel with AR-006/AR-007/AR-008