DeepSeek Harness Plugin: apply-patch

The `apply-patch` plugin fills a functional gap in DeepSeek Harness (DSH) by providing unified diff writing to disk. It supports full coverage of git diffs and features atomicity (all-or-nothing), precise reversal, dry-run mode, and binary safety. The plugin relies on no heavy libraries, strictly validates input through a self-implemented parser, and uses DSH file identifiers for version control. It provides four tools: `patch_apply`, `patch_dry_run`, `patch_reverse`, and `patch_stat`, and supports three usage modes: DSH, CLI, and library, ensuring that the diff application process is reliable and reversible.