docs: organisation security policy

SECURITY.md

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+# Security policy
+
+This policy applies to every repository in the PulseEngine organisation that does not have its own `SECURITY.md`.
+
+PulseEngine is a WebAssembly Component Model engine for safety-critical systems. It is work in progress, and no part of it is certified for production use. We still want to hear about security problems early.
+
+## Reporting a vulnerability
+
+Report it privately through GitHub:
+
+1. Open the affected repository on GitHub.
+2. Go to the **Security** tab and choose **Report a vulnerability**.
+3. Describe the problem, the affected version or commit, and how to reproduce it if you can.
+
+Please do **not** open a public issue, pull request or discussion for a vulnerability. If you are not sure which repository is affected, report it in the one that seems closest. We will move it if needed.
+
+## What to expect
+
+- We confirm receipt within a few working days. The project is run by a small team, so please allow for that.
+- We assess the report and keep you informed in the private advisory thread.
+- We fix the problem and publish a GitHub Security Advisory, with credit if you want it.
+
+## Scope
+
+In scope:
+
+- the engine and its tools (for example meld, loom, synth, kiln, sigil, rivet, spar, witness);
+- their release artifacts and CI workflows;
+- the `pulseengine-claude` plugin published from `pulseengine/pulseengine.eu`.
+
+Out of scope:
+
+- forks of third-party projects in this organisation. Report those upstream.