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.build.json

@@ -22,6 +22,7 @@
     { "path": "./packages/core/agent-loop" },
     { "path": "./packages/core/agent-core" },
     { "path": "./packages/bash/bash" },
+    { "path": "./packages/compact/compact" },
     { "path": "./packages/llm/llm-deepseek" },
     { "path": "./packages/llm/llm-pi-ai" },
     { "path": "./packages/bash/bash-local" },