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.

docs/module-graph.md

@@ -18,6 +18,8 @@ graph TD
   agent --> brand
   agent --> llm
   agent --> session
+  compact --> llm
+  compact --> session
   llm-replay --> llm
   llm-replay --> session
   session-persistence --> session
@@ -78,6 +80,7 @@ graph TD
 | `session` | `brand`, `llm` |
 | `system-prompt` | `llm` |
 | `agent` | `brand`, `llm`, `session` |
+| `compact` | `llm`, `session` |
 | `llm-replay` | `llm`, `session` |
 | `session-persistence` | `session` |
 | `invariants` | `agent`, `llm`, `session` |