feat(session): default JSONL writes to packed rows

scripts/migrate-packed-session-fixtures.ts

@@ -0,0 +1,21 @@
+#!/usr/bin/env node
+/**
+ * Temporary branch-convergence command for canonical packed session fixtures.
+ *
+ * @see ../.agents/notes/proposed/process/2026-07-26-remove-packed-session-fixture-migrator.md
+ */
+
+import { writeFileSync } from 'node:fs'
+import { resolve } from 'node:path'
+import { inspectSessionFixtureLayouts } from './session-fixture-layout.ts'
+
+if (process.argv.length > 2) throw new Error('migrate:packed-session-fixtures takes no arguments')
+
+const root = resolve(import.meta.dirname, '..')
+const fixtures = inspectSessionFixtureLayouts(root)
+const changed = fixtures.filter(fixture => fixture.source !== fixture.canonical)
+for (const fixture of changed) {
+  writeFileSync(resolve(root, fixture.path), fixture.canonical)
+  console.log(fixture.path)
+}
+console.log(`packed session fixtures: ${changed.length} rewritten, ${fixtures.length} inspected`)