docs(engine): clarify ENGINE_V2 opt-in startup

[serrrfirat]

Summary

• document that Engine v2 is opt-in and requires ENGINE_V2=true
• add concrete startup examples to the main README and localized READMEs
• add the same opt-in note to .env.example and the internal Engine v2 architecture doc

Testing

• not run (docs-only changes)

[gemini-code-assist]

Code Review

This pull request introduces documentation and configuration for the opt-in Engine v2, updating the .env.example file, main and localized READMEs, and internal architecture docs. The changes explain how to enable the new engine via the ENGINE_V2=true environment variable. Feedback highlights inconsistencies between the main README and its localized versions, specifically regarding missing usage examples for installed binaries and missing links to the architecture documentation.

[gemini-code-assist]

The main README.md is missing the examples for running the installed binary (ironclaw) which were added to all localized READMEs. For consistency and to help users who have installed the tool via Homebrew or other installers, please include them here as well.

Suggested change

	# Start interactive REPL with engine v2
	ENGINE_V2=true cargo run
	# Start installed binary
	ironclaw
	# Start installed binary with engine v2
	ENGINE_V2=true ironclaw
	# Start interactive REPL with engine v2
	ENGINE_V2=true cargo run

[gemini-code-assist]

This localized README is missing the link to the Engine v2 architecture documentation that was added to the main README.md. Adding it would help users find more detailed information about enabling the new engine.

Suggested change

	Engine v2 は現在オプトインです。従来のエージェントループではなく新しいエンジンを使いたい場合は、IronClaw を `ENGINE_V2=true` 付きで起動してください。
	Engine v2 は現在オプトインです。従来のエージェントループではなく新しいエンジンを使いたい場合は、IronClaw を ENGINE_V2=true 付きで起動してください。詳細は [Engine v2 architecture](docs/internal/engine-v2-architecture.md#enabling-engine-v2) を参照してください。

[gemini-code-assist]

This localized README is missing the link to the Engine v2 architecture documentation that was added to the main README.md. Adding it would help users find more detailed information about enabling the new engine.

Suggested change

	Engine v2는 현재 옵트인입니다. 기존 에이전트 루프 대신 새 엔진을 사용하려면 `ENGINE_V2=true`를 설정해서 IronClaw를 시작하세요.
	Engine v2는 현재 옵트인입니다. 기존 에이전트 루프 대신 새 엔진을 사용하려면 ENGINE_V2=true를 설정해서 IronClaw를 시작하세요. 자세한 내용은 [Engine v2 architecture](docs/internal/engine-v2-architecture.md#enabling-engine-v2)를 참조하세요.

[gemini-code-assist]

This localized README is missing the link to the Engine v2 architecture documentation that was added to the main README.md. Adding it would help users find more detailed information about enabling the new engine.

Suggested change

	Сейчас engine v2 включается отдельно. Если вы хотите запускать новый движок вместо устаревшего цикла агента, запускайте IronClaw с `ENGINE_V2=true`.
	Сейчас engine v2 включается отдельно. Если вы хотите запускать новый движок вместо устаревшего цикла агента, запускайте IronClaw с ENGINE_V2=true. Подробности см. в [Engine v2 architecture](docs/internal/engine-v2-architecture.md#enabling-engine-v2).

[gemini-code-assist]

This localized README is missing the link to the Engine v2 architecture documentation that was added to the main README.md. Adding it would help users find more detailed information about enabling the new engine.

Suggested change

	Engine v2 当前需要显式开启。如果你想运行新的引擎而不是旧的代理循环，请在启动 IronClaw 时设置 `ENGINE_V2=true`。
	Engine v2 当前需要显式开启。如果你想运行新的引擎而不是旧的代理循环，请在启动 IronClaw 时设置 ENGINE_V2=true。详见 [Engine v2 architecture](docs/internal/engine-v2-architecture.md#enabling-engine-v2)。