Harden read and bash output against binary data

[K-Mistele]

Summary

• reject known binary files, NUL bytes anywhere in a file, and malformed UTF-8 from the read tool with model-visible errors
• decode bash stdout and stderr as strict UTF-8 and replace unsafe stream output with a safe, model-visible omission notice
• cover binary reads, late NUL bytes, malformed UTF-8, and safe streamed bash deltas

Context

OpenCode's newer read implementation rejects binary content and uses fatal UTF-8 decoding. Pi and OpenCode's bash implementations currently use replacement decoding and can preserve NUL bytes; Fold now hardens both ingress paths so tool output cannot carry PostgreSQL-invalid NUL content into event persistence.

Validation

• bun run typecheck
• bun run format:check
• bun run lint (passes with existing warnings)
• bun run test