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/hybrid.ts

@@ -1,8 +1,14 @@
 import { applyMMRToHybridResults, type MMRConfig, DEFAULT_MMR_CONFIG } from "./mmr.js";
+import {
+  applyTemporalDecayToHybridResults,
+  type TemporalDecayConfig,
+  DEFAULT_TEMPORAL_DECAY_CONFIG,
+} from "./temporal-decay.js";
 
 export type HybridSource = string;
 
 export { type MMRConfig, DEFAULT_MMR_CONFIG };
+export { type TemporalDecayConfig, DEFAULT_TEMPORAL_DECAY_CONFIG };
 
 export type HybridVectorResult = {
   id: string;
@@ -42,21 +48,28 @@ export function bm25RankToScore(rank: number): number {
   return 1 / (1 + normalized);
 }
 
-export function mergeHybridResults(params: {
+export async function mergeHybridResults(params: {
   vector: HybridVectorResult[];
   keyword: HybridKeywordResult[];
   vectorWeight: number;
   textWeight: number;
+  workspaceDir?: string;
   /** MMR configuration for diversity-aware re-ranking */
   mmr?: Partial<MMRConfig>;
-}): Array<{
-  path: string;
-  startLine: number;
-  endLine: number;
-  score: number;
-  snippet: string;
-  source: HybridSource;
-}> {
+  /** Temporal decay configuration for recency-aware scoring */
+  temporalDecay?: Partial<TemporalDecayConfig>;
+  /** Test seam for deterministic time-dependent behavior */
+  nowMs?: number;
+}): Promise<
+  Array<{
+    path: string;
+    startLine: number;
+    endLine: number;
+    score: number;
+    snippet: string;
+    source: HybridSource;
+  }>
+> {
   const byId = new Map<
     string,
     {
@@ -117,7 +130,14 @@ export function mergeHybridResults(params: {
     };
   });
 
-  const sorted = merged.toSorted((a, b) => b.score - a.score);
+  const temporalDecayConfig = { ...DEFAULT_TEMPORAL_DECAY_CONFIG, ...params.temporalDecay };
+  const decayed = await applyTemporalDecayToHybridResults({
+    results: merged,
+    temporalDecay: temporalDecayConfig,
+    workspaceDir: params.workspaceDir,
+    nowMs: params.nowMs,
+  });
+  const sorted = decayed.toSorted((a, b) => b.score - a.score);
 
   // Apply MMR re-ranking if enabled
   const mmrConfig = { ...DEFAULT_MMR_CONFIG, ...params.mmr };