docs: add a security policy

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Please report security issues privately, not in public issues, pull requests or
+Discussions.
+
+Use the repository's [private vulnerability
+reporting](https://github.com/openagents-org/openagents/security/advisories/new)
+form. If that page is unavailable, email **team@openagents.org** with `SECURITY`
+in the subject.
+
+Please include:
+
+- what the issue is and which component it affects (network, workspace backend,
+  workspace frontend, launcher, SDK)
+- the version, tag or commit you tested against
+- steps to reproduce, ideally the smallest set that shows the problem
+- what an attacker gains, and what they need first (a workspace token, a
+  workspace id, an account somewhere else)
+
+You will get an acknowledgement within 5 working days. Please give us 90 days
+before public disclosure, or less if a fix ships sooner.
+
+## Scope
+
+In scope:
+
+- the network and its transports
+- the workspace backend and frontend
+- the launcher and its agent adapters
+- the published npm packages
+
+Out of scope:
+
+- vulnerabilities in a third-party agent runtime the launcher shells out to.
+  Report those to that project's maintainers.
+- anything requiring a workspace token you were already given. The token is the
+  credential for a workspace.
+- findings against `workspace.openagents.org` that only affect your own
+  workspace.
+
+## Notes for self-hosted deployments
+
+Two properties of the current design shape what does and does not count as a
+vulnerability.
+
+**A workspace token is a single shared credential.** Every agent in a workspace
+authenticates with the same token, and revocation is per workspace. Use one
+workspace per trust boundary rather than one workspace for everything.
+
+**Identity providers must be configured explicitly.** The identity-provider
+settings name the tenant a deployment trusts. Pointing them at a tenant you do
+not operate means accepting logins minted there. Leave them empty unless you
+own the tenant.
+
+## Supported Versions
+
+Fixes land on `develop` and ship in the next release. Only the latest release is
+supported; there are no backports to older tags.