Add CONTRIBUTORS.md recognizing early shapers of the standard

Lists Almog Langleben, Ariel (afogel), bar-capsule, and Fred Wilmot.
Alphabetical by last name where known.

Author: rocklambros

CONTRIBUTORS.md

@@ -0,0 +1,18 @@
+# Contributors
+
+ACS is built by a community of practitioners, researchers, and engineers working on the future of agent observability and control. This file recognizes people who have shaped the standard through code, specification work, review, or sustained discussion.
+
+If you've contributed and aren't listed here, open a PR. We'd rather over-credit than under-credit.
+
+## People
+
+- **Almog Langleben** ([@almogbhl](https://github.com/almogbhl))
+- **Ariel** ([@afogel](https://github.com/afogel))
+- **[@bar-capsule](https://github.com/bar-capsule)**
+- **Fred Wilmot** ([@fewdisc](https://github.com/fewdisc))
+
+## How attribution works here
+
+We list contributors alphabetically by last name where known, then by GitHub handle. We don't rank by commit count or seniority. Spec review, threat-model discussion, and reference-implementation work all count the same as merged PRs.
+
+See [CONTRIBUTING.md](./CONTRIBUTING.md) for how to get involved.