Fix export/import feature: JSON format and project filtering

Two critical fixes for the memory export/import feature:

1. **SearchManager empty results bug**: The /api/search endpoint
   with format=json now returns {observations: [], sessions: [],
   prompts: []} when no results are found, instead of the MCP
   protocol format. This fixes the export-memories script which
   expects consistent JSON structure regardless of result count.

2. **Project filtering support**: Updated SessionStore methods
   (getObservationsByIds, getSessionSummariesByIds, getUserPromptsByIds)
   to accept and apply project filter parameter. This enables
   proper project-based filtering during ChromaDB hybrid search
   result hydration.

Testing:
- Export with results: ✅ 50 observations exported
- Export with empty results: ✅ Proper JSON structure
- Round-trip import: ✅ Duplicate prevention working
- Project filtering: ✅ claude-mem (51 obs) vs rad-mem (1 obs)

Fixes export/import feature blocking bugs.

src/services/worker/SearchManager.ts

@@ -198,6 +198,13 @@ export class SearchManager {
         const totalResults = observations.length + sessions.length + prompts.length;
 
         if (totalResults === 0) {
+          if (format === 'json') {
+            return {
+              observations: [],
+              sessions: [],
+              prompts: []
+            };
+          }
           return {
             content: [{
               type: 'text' as const,
@@ -230,6 +237,15 @@ export class SearchManager {
         const limitedResults = allResults.slice(0, options.limit || 20);
 
         // Format based on requested format
+        if (format === 'json') {
+          // Raw JSON format for exports
+          return {
+            observations,
+            sessions,
+            prompts
+          };
+        }
+
         let combinedText: string;
         if (format === 'index') {
           const header = `Found ${totalResults} result(s) matching "${query}" (${observations.length} obs, ${sessions.length} sessions, ${prompts.length} prompts):\n\n`;