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>

src/utils/usage-logger.ts

@@ -0,0 +1,61 @@
+import { appendFileSync } from 'fs';
+import { join } from 'path';
+import { homedir } from 'os';
+
+/**
+ * Usage data structure from Claude Agent SDK result messages
+ */
+export interface UsageData {
+  timestamp: string;
+  sessionDbId: number;
+  claudeSessionId: string;
+  project: string;
+  promptNumber: number;
+  model: string;
+  sessionId: string; // SDK session ID
+  uuid: string; // SDK message UUID
+  durationMs: number;
+  durationApiMs: number;
+  numTurns: number;
+  totalCostUsd: number;
+  usage: {
+    inputTokens: number;
+    outputTokens: number;
+    cacheCreationInputTokens: number;
+    cacheReadInputTokens: number;
+  };
+}
+
+/**
+ * Logger for capturing usage metrics to JSONL files
+ */
+export class UsageLogger {
+  private logDir: string;
+  private logFile: string;
+
+  constructor() {
+    this.logDir = join(homedir(), '.claude-mem', 'usage-logs');
+    // Create a daily log file
+    const date = new Date().toISOString().split('T')[0]; // YYYY-MM-DD
+    this.logFile = join(this.logDir, `usage-${date}.jsonl`);
+  }
+
+  /**
+   * Log usage data from SDK result message
+   */
+  logUsage(data: UsageData): void {
+    try {
+      const line = JSON.stringify(data) + '\n';
+      appendFileSync(this.logFile, line, 'utf-8');
+    } catch (error) {
+      console.error('Failed to log usage data:', error);
+    }
+  }
+
+  /**
+   * Get the current log file path
+   */
+  getLogFilePath(): string {
+    return this.logFile;
+  }
+}