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/temporal-decay.ts

@@ -0,0 +1,166 @@
+import fs from "node:fs/promises";
+import path from "node:path";
+
+export type TemporalDecayConfig = {
+  enabled: boolean;
+  halfLifeDays: number;
+};
+
+export const DEFAULT_TEMPORAL_DECAY_CONFIG: TemporalDecayConfig = {
+  enabled: false,
+  halfLifeDays: 30,
+};
+
+const DAY_MS = 24 * 60 * 60 * 1000;
+const DATED_MEMORY_PATH_RE = /(?:^|\/)memory\/(\d{4})-(\d{2})-(\d{2})\.md$/;
+
+export function toDecayLambda(halfLifeDays: number): number {
+  if (!Number.isFinite(halfLifeDays) || halfLifeDays <= 0) {
+    return 0;
+  }
+  return Math.LN2 / halfLifeDays;
+}
+
+export function calculateTemporalDecayMultiplier(params: {
+  ageInDays: number;
+  halfLifeDays: number;
+}): number {
+  const lambda = toDecayLambda(params.halfLifeDays);
+  const clampedAge = Math.max(0, params.ageInDays);
+  if (lambda <= 0 || !Number.isFinite(clampedAge)) {
+    return 1;
+  }
+  return Math.exp(-lambda * clampedAge);
+}
+
+export function applyTemporalDecayToScore(params: {
+  score: number;
+  ageInDays: number;
+  halfLifeDays: number;
+}): number {
+  return params.score * calculateTemporalDecayMultiplier(params);
+}
+
+function parseMemoryDateFromPath(filePath: string): Date | null {
+  const normalized = filePath.replaceAll("\\", "/").replace(/^\.\//, "");
+  const match = DATED_MEMORY_PATH_RE.exec(normalized);
+  if (!match) {
+    return null;
+  }
+
+  const year = Number(match[1]);
+  const month = Number(match[2]);
+  const day = Number(match[3]);
+  if (!Number.isInteger(year) || !Number.isInteger(month) || !Number.isInteger(day)) {
+    return null;
+  }
+
+  const timestamp = Date.UTC(year, month - 1, day);
+  const parsed = new Date(timestamp);
+  if (
+    parsed.getUTCFullYear() !== year ||
+    parsed.getUTCMonth() !== month - 1 ||
+    parsed.getUTCDate() !== day
+  ) {
+    return null;
+  }
+
+  return parsed;
+}
+
+function isEvergreenMemoryPath(filePath: string): boolean {
+  const normalized = filePath.replaceAll("\\", "/").replace(/^\.\//, "");
+  if (normalized === "MEMORY.md" || normalized === "memory.md") {
+    return true;
+  }
+  if (!normalized.startsWith("memory/")) {
+    return false;
+  }
+  return !DATED_MEMORY_PATH_RE.test(normalized);
+}
+
+async function extractTimestamp(params: {
+  filePath: string;
+  source?: string;
+  workspaceDir?: string;
+}): Promise<Date | null> {
+  const fromPath = parseMemoryDateFromPath(params.filePath);
+  if (fromPath) {
+    return fromPath;
+  }
+
+  // Memory root/topic files are evergreen knowledge and should not decay.
+  if (params.source === "memory" && isEvergreenMemoryPath(params.filePath)) {
+    return null;
+  }
+
+  if (!params.workspaceDir) {
+    return null;
+  }
+
+  const absolutePath = path.isAbsolute(params.filePath)
+    ? params.filePath
+    : path.resolve(params.workspaceDir, params.filePath);
+
+  try {
+    const stat = await fs.stat(absolutePath);
+    if (!Number.isFinite(stat.mtimeMs)) {
+      return null;
+    }
+    return new Date(stat.mtimeMs);
+  } catch {
+    return null;
+  }
+}
+
+function ageInDaysFromTimestamp(timestamp: Date, nowMs: number): number {
+  const ageMs = Math.max(0, nowMs - timestamp.getTime());
+  return ageMs / DAY_MS;
+}
+
+export async function applyTemporalDecayToHybridResults<
+  T extends { path: string; score: number; source: string },
+>(params: {
+  results: T[];
+  temporalDecay?: Partial<TemporalDecayConfig>;
+  workspaceDir?: string;
+  nowMs?: number;
+}): Promise<T[]> {
+  const config = { ...DEFAULT_TEMPORAL_DECAY_CONFIG, ...params.temporalDecay };
+  if (!config.enabled) {
+    return [...params.results];
+  }
+
+  const nowMs = params.nowMs ?? Date.now();
+  const timestampCache = new Map<string, Date | null>();
+
+  return Promise.all(
+    params.results.map(async (entry) => {
+      const cacheKey = `${entry.source}:${entry.path}`;
+      if (!timestampCache.has(cacheKey)) {
+        const timestamp = await extractTimestamp({
+          filePath: entry.path,
+          source: entry.source,
+          workspaceDir: params.workspaceDir,
+        });
+        timestampCache.set(cacheKey, timestamp);
+      }
+
+      const timestamp = timestampCache.get(cacheKey) ?? null;
+      if (!timestamp) {
+        return entry;
+      }
+
+      const decayedScore = applyTemporalDecayToScore({
+        score: entry.score,
+        ageInDays: ageInDaysFromTimestamp(timestamp, nowMs),
+        halfLifeDays: config.halfLifeDays,
+      });
+
+      return {
+        ...entry,
+        score: decayedScore,
+      };
+    }),
+  );
+}