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.

pnpm-lock.yaml

@@ -118,6 +118,18 @@ importers:
         specifier: ^4.0.0-rc.6
         version: 4.0.0-rc.6(@cordisjs/plugin-include@1.0.4)(@cordisjs/plugin-loader@1.0.0-rc.4)
 
+  packages/compact/compact:
+    devDependencies:
+      '@deepseek-ai/dsh-llm':
+        specifier: workspace:^
+        version: link:../../llm/llm
+      '@deepseek-ai/dsh-session':
+        specifier: workspace:^
+        version: link:../../core/session
+      cordis:
+        specifier: ^4.0.0-rc.6
+        version: 4.0.0-rc.6(@cordisjs/plugin-include@1.0.4)(@cordisjs/plugin-loader@1.0.0-rc.4)
+
   packages/core/agent:
     devDependencies:
       '@deepseek-ai/dsh-brand':