Lequait/dsh-context-rescue

Recovers a session the host already refused for exceeding the context window: it re-checks the gateway message the built-in classifier misses, takes the real window out of that message, and folds the oldest span into one checkpoint without a model call (tool-result pruning, the host compaction engine when it can shrink the projection, or a mechanical digest fold), then retries the failed step; a context_rescue tool and /rescue, /rescue-status expose the same path by hand.