DeepSeek Harness File Validation Plugin

`dsh-file-checksum` is a secure read-only plugin for DeepSeek Harness (DSH) that computes raw SHA-256 or SHA-512 checksums without exposing file contents. It reads files via `ctx.fs` and returns only structured hash values and validation status (match/mismatch), preventing leakage of sensitive data. Installation requires the DSH plugin command, provides the `file_checksum` tool, supports single-file checksum validation and verification using the `expected` parameter. Configuration can limit the maximum size of a single file (default 64MB). The plugin offers security features such as read-only operation, no Shell invocation, concurrency control, and integrity logging. It depends on DSH version 0.1.0-rc.6 and is suitable for building verification scenarios.