docs: add SECURITY.md with a private reporting path

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+**Please do not open a public issue for a security problem.**
+
+Report it privately through GitHub:
+[**Report a vulnerability**](https://github.com/StarTrail-org/LEANN/security/advisories/new)
+(repository → **Security** → **Advisories** → *Report a vulnerability*).
+
+That opens a private advisory visible only to you and the maintainers. It needs
+no infrastructure on either side, and it gives us a place to discuss a fix and
+issue a CVE if one is warranted.
+
+If the advisory form is unavailable to you, open a public issue containing only
+that you have a security report and how to reach you — no details — and a
+maintainer will arrange a private channel.
+
+## What to include
+
+The more of this you can provide, the faster a fix lands:
+
+- what an attacker can achieve, and what access they need to start
+- affected versions, and the platform and Python version you saw it on
+- a reproduction — a script, or the commands you ran
+- anything you already know about the cause or a possible fix
+
+## Scope
+
+LEANN runs locally and is usually pointed at a user's own data, so the
+interesting boundaries are the ones where it stops being local:
+
+- the backend embedding servers, which speak an **unauthenticated ZMQ REP
+  protocol** — they bind `127.0.0.1` by default, and anything that can reach
+  the port can request embeddings (see
+  [Embedding Server Bind Address](docs/configuration-guide.md))
+- the MCP server and its stdio transport
+- document readers and parsers, which handle untrusted input by design
+- index and metadata files written to disk, and anything that reads a path out
+  of them
+- credentials for third-party providers — API keys, endpoints, tokens
+
+Out of scope: findings against a deployment you have deliberately exposed to a
+network (for example `LEANN_EMBEDDING_SERVER_HOST=0.0.0.0` reachable from the
+internet), and vulnerabilities in third-party dependencies that are already
+public — please report those upstream, though we do want to know if LEANN
+pins an affected version.
+
+## Supported versions
+
+Fixes land on `main` and ship in the next release. If you are on an older
+release, please confirm the issue reproduces on current `main` where you can.
+
+## Disclosure
+
+We will confirm receipt, keep you updated as we investigate, and credit you in
+the advisory unless you would rather stay anonymous. Please give us a
+reasonable window to ship a fix before disclosing publicly.