refactor(retention): extract retention module from workspace-memory

Move retention constants and math to a focused src/retention.ts module:
- All half-life, reinforcement, dormancy constants
- TYPE_FACTOR, SOURCE_FACTOR, USER_IMPORTANCE_FACTOR
- RETENTION_TYPE_MAX (renamed from TYPE_MAX)
- calculateInitialStrength, calculateEffectiveHalfLife,
  calculateRetentionStrength, calculateDormantDays,
  calculateEffectiveAgeDays, reinforceMemory

No behavior changes. retention.ts imports only types from types.ts.
Workspace-memory.ts still owns storage, consolidation, and rendering.

src/plugin.ts

@@ -36,8 +36,8 @@ import {
   updateWorkspaceMemory,
   updateWorkspaceMemoryWithAccounting,
   renderWorkspaceMemory,
-  reinforceMemory,
 } from "./workspace-memory.ts";
+import { reinforceMemory } from "./retention.ts";
 import {
   appendPendingMemories,
   clearPendingMemories,