feat: add MultiActorSession for coordinated multi-user browser testing

[drewstone]

Summary

• Adds MultiActorSession and Actor classes for coordinated multi-user browser testing
• Each actor gets an isolated BrowserContext (separate cookies, localStorage, auth state) while sharing the same Browser instance
• Imperative API — consumers orchestrate with standard async/await for sequential flows and Promise.all() / session.parallel() for concurrent execution
• Cleanup-safe: partial creation failures (e.g., setup hook throws) close all already-created contexts
• 22 unit tests covering all public API surface, error paths, and edge cases

Usage

const session = await MultiActorSession.create(browser, {
  actors: {
    admin:   { storageState: '.auth/admin.json' },
    partner: { storageState: '.auth/partner.json' },
    user1:   {},
  },
  agentConfig: { model: 'gpt-4o', vision: true },
});

// Sequential
await session.actor('admin').run({ goal: 'Create quest', startUrl: '/admin' });
await session.actor('partner').run({ goal: 'Approve quest' });

// Parallel
await session.parallel(
  ['user1', { goal: 'Start quest' }],
  ['admin', { goal: 'Monitor dashboard' }],
);

await session.close();

Test plan

• npx tsc --noEmit — zero type errors
• npx vitest run tests/multi-actor.test.ts — 22/22 pass
• npx vitest run — 79/79 pass (zero regressions)
• npx tsc — dist output includes multi-actor.{js,d.ts}

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