Improve the UX preview plan project

[nturinski]

New preview:

[MicroFish91]

@copilot Can you provide a summary of the changes here, and what the overall goals of the PR are?

[MicroFish91]

Seeing how well Copilot can handle this... for research purposes 🤓

[Copilot]

@copilot Can you provide a summary of the changes here, and what the overall goals of the PR are?

This PR has two main parts:

1. A large scaffolding update that adds the Copilot-on-Rails planning assets (agent definitions, instructions, shared references, and related project/config/docs files) to support the frontend preview planning flow.
2. A follow-up cleanup commit that removes the custom esbuild-problem-matcher watch plugin from esbuild.copilotOnRailsViews.mjs.

Overall goal: improve the project preview planning UX by giving the planning/scaffolding experience better structure and guidance, while simplifying the watch/build setup to make local iteration more predictable.