docs(agents): refocus contributor guide

[onutc]

TL;DR

"agents md shouldn't contain so much technical docs. it should contain info about how to deveop, build, install, test, ci/cd, when to run tests, github repo, etc."
"contain important parts from the vision doc verbatim inside agents md, like scrutinizing, overall vision, it should be thin and such"

Summary

• Rewrite AGENTS.md around contributor workflow: setup, local development, build, install, testing, CI, release, and key repo references.
• Keep the important product-direction constraints in the file verbatim from VISION.md.
• Link readers to the actual in-repo technical references instead of keeping AGENTS.md as a long technical spec.

Review focus

• Confirm the new AGENTS.md strikes the right balance between contributor workflow and product-direction guardrails.
• Confirm the linked technical references are the right ones to send contributors to for implementation details.

Test plan

• pnpm exec oxfmt --check AGENTS.md
• pnpm exec markdownlint-cli2 AGENTS.md
• git commit hook completed a local build successfully
• Full pnpm run check not run locally