feat(memory): Add opt-in temporal decay for hybrid search scoring

Exponential decay (half-life configurable, default 30 days) applied
before MMR re-ranking. Dated daily files (memory/YYYY-MM-DD.md) use
filename date; evergreen files (MEMORY.md, topic files) are not
decayed; other sources fall back to file mtime.

Config: memorySearch.query.hybrid.temporalDecay.{enabled, halfLifeDays}
Default: disabled (backwards compatible, opt-in).

src/memory/manager.ts

@@ -278,11 +278,13 @@ export class MemoryIndexManager implements MemorySearchManager {
       return vectorResults.filter((entry) => entry.score >= minScore).slice(0, maxResults);
     }
 
-    const merged = this.mergeHybridResults({
+    const merged = await this.mergeHybridResults({
       vector: vectorResults,
       keyword: keywordResults,
       vectorWeight: hybrid.vectorWeight,
       textWeight: hybrid.textWeight,
+      mmr: hybrid.mmr,
+      temporalDecay: hybrid.temporalDecay,
     });
 
     return merged.filter((entry) => entry.score >= minScore).slice(0, maxResults);
@@ -343,8 +345,10 @@ export class MemoryIndexManager implements MemorySearchManager {
     keyword: Array<MemorySearchResult & { id: string; textScore: number }>;
     vectorWeight: number;
     textWeight: number;
-  }): MemorySearchResult[] {
-    const merged = mergeHybridResults({
+    mmr?: { enabled: boolean; lambda: number };
+    temporalDecay?: { enabled: boolean; halfLifeDays: number };
+  }): Promise<MemorySearchResult[]> {
+    return mergeHybridResults({
       vector: params.vector.map((r) => ({
         id: r.id,
         path: r.path,
@@ -365,8 +369,10 @@ export class MemoryIndexManager implements MemorySearchManager {
       })),
       vectorWeight: params.vectorWeight,
       textWeight: params.textWeight,
-    });
-    return merged.map((entry) => entry as MemorySearchResult);
+      mmr: params.mmr,
+      temporalDecay: params.temporalDecay,
+      workspaceDir: this.workspaceDir,
+    }).then((entries) => entries.map((entry) => entry as MemorySearchResult));
   }
 
   async sync(params?: {