Add AGENTS.md, CODEOWNERS, and PR template for agent-first readiness

[miguelcalderon]

Closes https://nutrient.atlassian.net/browse/WEB-4154

What

Adds agent-first infrastructure to make this repo discoverable and contribution-friendly for both AI agents and human contributors.

Changes

• AGENTS.md — Documents repo structure, content types (playground snippet vs local example vs external link), contribution workflows, commands, and code style rules
• CODEOWNERS — Assigns Dev Rel as default reviewers, Web team as backup for web/ and playground/ directories
• PR template — Standardized checklist for contributions (type, formatting, README updates, testing)

Context

Part of the agent-first repos initiative discussed in #engineering.

Refs: https://nutrient.atlassian.net/browse/AI-451
Closes https://nutrient.atlassian.net/browse/WEB-4154

[miguelcalderon]

@iperzic Thanks, good catches! 🙈

I’ll fix the CODEOWNERS handle.

I also agree on AGENTS.md: it should be a navigation doc, not a second copy of repo structure. Looking at the current repo, the root README.md, playground/README.md, category READMEs under web/, and per-example package.json files already carry most of the structure and run instructions.

I’m going to trim AGENTS.md down so it points to those sources of truth and keeps only the repo-specific guidance that isn’t obvious from inspection: mainly contribution type selection, discoverability expectations, and where to update indexes/READMEs.

Let me know your thoughts. Thanks!

[iperzic]

Much better, thank you!