feat: sync ecc2 cost tracker metrics

tests/hooks/cost-tracker.test.js

@@ -131,6 +131,27 @@ function runTests() {
     fs.rmSync(tmpHome, { recursive: true, force: true });
   }) ? passed++ : failed++);
 
+  // 6. Prefers ECC_SESSION_ID for ECC2 session correlation
+  (test('prefers ECC_SESSION_ID over CLAUDE_SESSION_ID when both are present', () => {
+    const tmpHome = makeTempDir();
+    const input = {
+      model: 'claude-sonnet-4-20250514',
+      usage: { input_tokens: 120, output_tokens: 30 },
+    };
+    const result = runScript(input, {
+      ...withTempHome(tmpHome),
+      ECC_SESSION_ID: 'ecc-session-1234',
+      CLAUDE_SESSION_ID: 'claude-session-9999',
+    });
+    assert.strictEqual(result.code, 0, `Expected exit code 0, got ${result.code}`);
+
+    const metricsFile = path.join(tmpHome, '.claude', 'metrics', 'costs.jsonl');
+    const row = JSON.parse(fs.readFileSync(metricsFile, 'utf8').trim());
+    assert.strictEqual(row.session_id, 'ecc-session-1234', 'Expected ECC_SESSION_ID to win');
+
+    fs.rmSync(tmpHome, { recursive: true, force: true });
+  }) ? passed++ : failed++);
+
   console.log(`\nResults: Passed: ${passed}, Failed: ${failed}`);
   process.exit(failed > 0 ? 1 : 0);
 }