feat: Add cross-encoder re-ranking after hybrid search (CF-1317)

Add rerank() function calling LiteLLM /v1/rerank endpoint (Cohere-compatible).
Plugged into all 3 search functions (sessions, session-docs, archives) after
RRF merge. Disabled by default via RERANK_ENABLED env var. Graceful fallback
to RRF-only ranking on API failure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/tools/archives.ts

@@ -1,7 +1,7 @@
 // Project archives operations for database-backed archival
 
 import { query, queryOne, execute } from '../db.js';
-import { getEmbedding, formatEmbedding, generateContentHash, rrfMerge } from '../embeddings.js';
+import { getEmbedding, formatEmbedding, generateContentHash, rrfMerge, rerank } from '../embeddings.js';
 
 type ArchiveType = 'session' | 'research' | 'audit' | 'investigation' | 'completed' | 'migration';
 
@@ -193,10 +193,30 @@ export async function archiveSearch(args: ArchiveSearchArgs): Promise<string> {
   let finalIds: number[];
   let searchLabel: string;
 
+  let rerankScores: Map<number, number> | null = null;
+
   if (search_mode === 'hybrid' && vectorIds.length > 0 && keywordIds.length > 0) {
     const merged = rrfMerge(vectorIds, keywordIds);
-    finalIds = merged.slice(0, limit).map(m => m.id as number);
+    finalIds = merged.map(m => m.id as number);
     searchLabel = 'hybrid';
+
+    // Cross-encoder re-ranking (CF-1317)
+    const docs = finalIds.map(id => {
+      const r = vectorRows.get(id) || keywordRows.get(id);
+      return (r as any)?.title || '';
+    });
+    const reranked = await rerank(searchQuery, docs, limit);
+    if (reranked) {
+      rerankScores = new Map();
+      const reorderedIds = reranked.map(r => {
+        rerankScores!.set(finalIds[r.index], r.relevance_score);
+        return finalIds[r.index];
+      });
+      finalIds = reorderedIds;
+      searchLabel = 'hybrid+rerank';
+    } else {
+      finalIds = finalIds.slice(0, limit);
+    }
   } else if (vectorIds.length > 0) {
     finalIds = vectorIds;
     searchLabel = 'vector';
@@ -212,9 +232,12 @@ export async function archiveSearch(args: ArchiveSearchArgs): Promise<string> {
   for (const id of finalIds) {
     const a = vectorRows.get(id) || keywordRows.get(id);
     if (!a) continue;
-    const sim = vectorRows.has(id) ? ` (${Math.round((vectorRows.get(id)!).similarity * 100)}% match)` : '';
+    const simParts: string[] = [];
+    if (vectorRows.has(id)) simParts.push(`${Math.round((vectorRows.get(id)!).similarity * 100)}% match`);
+    if (rerankScores?.has(id)) simParts.push(`rerank: ${rerankScores.get(id)!.toFixed(2)}`);
+    const scores = simParts.length > 0 ? ` (${simParts.join(', ')})` : '';
     const sizeStr = a.file_size ? ` (${Math.round(a.file_size / 1024)}KB)` : '';
-    lines.push(`**[${a.archive_type}]** ${a.title}${sim}`);
+    lines.push(`**[${a.archive_type}]** ${a.title}${scores}`);
     lines.push(`  Archived: ${a.archived_at}${sizeStr}`);
     if (a.original_path) {
       lines.push(`  Path: ${a.original_path}`);