#4010 docs: point stable test suites to e2e

Summary

The introduction's "Where it fits" section listed other test frameworks for stable suites. It now points to e2e, a TypeScript testing framework for web and mobile apps, and keeps the sentence about when to use agent-device. Scope expansion: none.

Validation

• pnpm check:affected --run at f5d390bc83: docs-links and docs-prose passed (exactCommit=true).
• Docs-only; not device-facing.

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