dsh-failbook: Adding a failure ledger to DeepSeek Harness

dsh-failbook is a plugin for DeepSeek Harness (DSH) designed to address invalid retries caused by tool call failures during agent runtime. It prevents duplicate error logs resulting from parameter changes by recording failure reasons and clustering them by signature, and injects alerts when the same failure signature recurs to guide the model to avoid blind retries. The plugin provides a Web UI panel that supports viewing top failure signatures, muting false positives, and manually clearing records. Core features include automatic recording of structured errors, cross-session persistent storage (falling back to an in-memory ledger when no storage service is available), and failure-aware interception. Configuration is flexible, allowing adjustments to retry thresholds, cooldown times, and excluded tools, with a default conservative detection principle to minimize false positives. Installation is simple, supporting command-line or manual configuration, and is MIT licensed and ready to use out of the box. Its design does not modify the tool...