docs: add an org-wide security policy

SECURITY.md

@@ -0,0 +1,57 @@
+# Security Policy
+
+This policy applies to all repositories in the [muxlang](https://github.com/muxlang)
+organization (the compiler, runtime, website, playground API, and tooling).
+
+## Supported versions
+
+Mux is in **alpha** and moves fast. Security fixes land on `main` and ship in the
+next release; only the latest release of each repository is supported. There are
+no long-term-support branches. The compiler version reported by `mux --version`
+is the canonical "Mux version".
+
+## Reporting a vulnerability
+
+**Please do not open a public issue for a security problem, and do not post
+details in discussions or pull requests.**
+
+Report privately through GitHub's private vulnerability reporting:
+
+1. Go to the affected repository's **Security** tab.
+2. Click **Report a vulnerability** (under "Advisories").
+3. Describe the issue with enough detail to reproduce it.
+
+This opens a private advisory visible only to you and the repository's
+maintainer team (the primary channel, so a report does not depend on any single
+person being available). If you cannot use private reporting, contact a
+maintainer - currently [@DerekCorniello](https://github.com/DerekCorniello) -
+directly and privately.
+
+Please include, where you can:
+
+- the repository and version or commit affected,
+- a description of the vulnerability and its impact,
+- steps or a minimal case to reproduce it,
+- any relevant logs, `.mux` input, or generated IR.
+
+## What to expect
+
+- We aim to acknowledge a report within a few days.
+- We will confirm the issue, keep you updated on progress, and credit you in the
+  advisory and release notes unless you ask us not to.
+- We aim to release a fix and publish the advisory within **90 days** of
+  acknowledgment. If we need longer, we will say so and agree a timeline with
+  you; if you plan to disclose publicly, please coordinate with us first so a
+  fix is available to users.
+
+## Scope
+
+The Mux toolchain compiles and runs untrusted code by design (notably the
+playground API, which executes user-submitted programs in a sandbox). Reports
+that are especially valuable include:
+
+- sandbox escapes or resource-exhaustion bypasses in the playground API,
+- compiler or runtime memory-safety issues reachable from valid Mux input,
+- vulnerable dependencies with a practical exploit path in a shipped artifact.
+
+Thank you for helping keep Mux and its users safe.