dsh-context-truth: Let the Model Know the Actual Context Usage

The DSH plugin `dsh-context-truth` resolves model non-convergence in long sessions caused by misjudged context state, using measured-usage backfill and compaction instruction rule patching. It displays accurate used/remaining token counts and the compaction threshold to the model, and prevents the model from writing its own statements about context exhaustion into summaries. It also provides idle-boundary folding to prevent the model from avoiding critical operations when nearing the threshold. The plugin depends on the host `tokenMeter` and `compaction`, requires no build, and is version 0.2.0.