[codex] Add MCP tool layer and refine handoff boundaries

[ACCS-0521]

Summary

• Expose ecommerce tools through a standard MCP server/client layer while reusing existing JSON-backed business tools.
• Refine handoff boundaries so ordinary service questions are answered by the agent instead of being sent to human support.
• Let the model compose boundary/identity/store-scope replies from guardrail context instead of hard-coded responses.
• Keep the chat input focused after each send for smoother frontend use.

Validation

• pnpm test
• pnpm run build
• pnpm --filter ecommerce-agent-web test
• pnpm --filter ecommerce-agent-web build