Include parent id in child launch prompts

[selfcontained]

Summary

• Prepend server-generated parent-agent context to generic dispatch_launch_agent child startup prompts
• Preserve the original caller prompt after the parent-id context
• Add handler coverage verifying the parent id and original prompt are passed into createAgent

Validation

• pnpm vitest run apps/server/test/mcp-handlers.test.ts
• pnpm run check
• pnpm run test:e2e
• pnpm run test
• Pre-commit hook: pnpm run check