chore: add local dev environment and agent notes

[juliusknorr]

• Target version: main

Summary

Adds a one-command local dev environment and an AGENTS.md describing how to use it to verify changes.

• npm run dev:server starts Nextcloud (server master) with this checkout mounted on http://localhost:8081 plus Collabora on http://localhost:9980, using @nextcloud/e2e-test-server
• Static files are served uncached, so npm run watch plus a reload shows changes immediately
• AGENTS.md lists the checks to run and how to verify frontend changes in a browser (Playwright MCP)

Stacked on #6142, as the dev server targets server master where the new viewer lives.

TODO

• Rebase onto main once fix: migrate to @nextcloud/viewer 2 and Vue 3 #6142 is merged

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required

[elzody]

Is the use case for this just so that agents can autonomously verify their changes and interact with a local dev setup? Or is there something else, like is this a good prerequisite for eventually migrating tests to Playwright?

[juliusknorr]

Both actually. The server setup can be reused for playwright but I use this to ask my agent in combination with playwright-mcp to test and verify changes or to do screenshots automatically.