feat(keg): add atomic schema-aware mutation APIs

[jlrickert]

Summary

• add atomic batch mutations and strict schema-policy enforcement across local, remote, CLI, and MCP surfaces
• add explicit schema-aware direct writes and advisory schema validation previews
• move RemoteKeg content and metadata replacements to aggregate JSON mutation endpoints
• keep schema policy, snapshots, dex updates, and transaction behavior consistent across write paths

Verification

• go test ./...
• go vet ./...
• go test -race ./...