feat(compact): compaction capability seam — abstract CompactService interface

Adds the @deepseek-ai/dsh-compact interface package: the abstract
CompactService (ctx.compact) with compactIfNeeded / compactRegion, the
compact/* session-event types via SessionEventMap declaration merging, and the
capability-seam RFC. Wires the package into the three root tsconfigs and the
cordis catalog. A backend implementation lands separately.

tsconfig.base.json

@@ -43,6 +43,7 @@
         "./packages/core/*/src",
         "./packages/llm/*/src",
         "./packages/bash/*/src",
+        "./packages/compact/*/src",
         "./packages/session-persistence/*/src",
         "./packages/ui/*/src",
         "./packages/util/*/src",