Add SDK usage tracking to JSONL logs

Features:
- New UsageLogger utility that writes usage metrics to daily JSONL files
- Captures token counts, costs, timing, and cache metrics from SDK result messages
- Usage logs stored in ~/.claude-mem/usage-logs/ (one file per day)
- Added analyze-usage.js script for analyzing usage patterns

Usage data captured:
- Token counts (input, output, cache creation, cache read)
- Total cost in USD per API call
- Duration metrics (total and API time)
- Number of turns per session
- Session and project attribution

Analysis script features:
- Aggregates totals by project and model
- Shows cache hit rates and savings
- Displays cost breakdowns and averages
- npm scripts: usage:analyze and usage:today

Files:
- src/utils/usage-logger.ts (new)
- src/services/worker-service.ts (modified - captures SDK result messages)
- scripts/analyze-usage.js (new)
- package.json (added usage:* npm scripts)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

context/claude-code/agent-sdk-cost-tracking.md

@@ -0,0 +1,61 @@
+ For tracking costs and tokens in your Agent SDK plugin, you have built-in programmatic access to usage data through the SDK itself[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking).
+
+## Agent SDK Cost Tracking
+
+The Claude Agent SDK provides detailed token usage information for each interaction[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking). Here's how to track it:
+
+**TypeScript:**
+```typescript
+import { query } from "@anthropic-ai/claude-agent-sdk";
+
+const result = await query({
+  prompt: "Your task here",
+  options: {
+    onMessage: (message) => {
+      if (message.type === 'assistant' && message.usage) {
+        console.log(`Message ID: ${message.id}`);
+        console.log(`Usage:`, message.usage);
+      }
+    }
+  }
+});
+```
+[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking)
+
+The final `result` message contains the total cumulative usage from all steps in the conversation[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking):
+
+```typescript
+console.log("Total usage:", result.usage);
+console.log("Total cost:", result.usage.total_cost_usd);
+```
+[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking)
+
+## Important: Avoid Double-Counting
+
+When Claude executes tools in parallel, multiple assistant messages may share the same ID and usage data[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking). You should only charge once per unique message ID[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking):
+
+```typescript
+const processedMessageIds = new Set<string>();
+
+onMessage: (message) => {
+  if (message.type === 'assistant' && message.usage) {
+    // Skip if already processed
+    if (processedMessageIds.has(message.id)) {
+      return;
+    }
+    
+    processedMessageIds.add(message.id);
+    // Record usage here
+  }
+}
+```
+[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking)
+
+## Usage Fields
+
+Each usage object contains[(1)](https://docs.claude.com/en/api/agent-sdk/cost-tracking):
+- `input_tokens`: Base input tokens processed
+- `output_tokens`: Tokens generated in the response
+- `cache_creation_input_tokens`: Tokens used to create cache entries
+- `cache_read_input_tokens`: Tokens read from cache
+- `total_cost_usd`: Total cost in USD (only in result message)