Add ADOPTERS.md with Nobulex as first adopter

Creates ADOPTERS.md as referenced in the README. Nobulex's bilateral receipt primitive was merged into AGT via PRs microsoft#1302 and microsoft#1333 (216 LOC, 11 tests). 8 independent implementations have byte-match validated the JCS/RFC 8785 canonicalization substrate. AAIF Growth-stage proposal filed with TC sponsorship from the CTEF maintainer.

Author: arian-gogani

ADOPTERS.md

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+# Adopters
+
+Organizations and projects using the Agent Governance Toolkit in production or as part of their agent security infrastructure.
+
+If you are using AGT, please consider adding yourself to this list by opening a pull request. It helps the project gain momentum and helps others discover real-world use cases.
+
+| Organization | Description | Usage | Links |
+|---|---|---|---|
+| [Nobulex](https://nobulex.com) | Cryptographic accountability infrastructure for AI agents | Bilateral receipt primitive merged into AGT (PRs [#1302](https://github.com/microsoft/agent-governance-toolkit/pull/1302), [#1333](https://github.com/microsoft/agent-governance-toolkit/pull/1333); 216 LOC, 11 tests). Pre-execution + post-execution Ed25519 signatures hash-chained via JCS/RFC 8785 for tamper-evident audit trails. 8 byte-match validated implementations across 7 organizations. AAIF Growth-stage project with TC sponsorship. | [GitHub](https://github.com/arian-gogani/nobulex) |