Add Mastra Harness Chat example

[vishxrad]

Summary

• Add a new examples/harnesses/mastra-harness Next.js example that connects OpenUI chat to Mastra's Harness API.
• Configure one default Harness mode with safe mock tools, LibSQL-backed Harness state, and an AG-UI event bridge for OpenUI rendering.
• Add localStorage-backed OpenUI thread persistence while keeping the UI on the stock OpenUI <FullScreen /> chat surface.
• Update the app, metadata, and README copy around the simplified Mastra Harness Chat experience.

Validation

• PATH=/opt/homebrew/bin:$PATH /opt/homebrew/bin/pnpm --filter mastra-harness run lint
• rm -rf examples/harnesses/mastra-harness/.next && PATH=/opt/homebrew/bin:$PATH /opt/homebrew/bin/pnpm --filter mastra-harness run typecheck
• PATH=/opt/homebrew/bin:$PATH /opt/homebrew/bin/pnpm --filter mastra-harness run build

Note: the local ignored .env.local was redacted before publishing; it is not included in this PR.