Add SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+Please do not put vulnerability details in a public issue.
+
+1. If the **Security** tab of this repository offers **"Report a vulnerability"**
+   (GitHub's private vulnerability reporting), use it.
+2. If that option is not available, open a public issue containing only the
+   words **"security contact requested"** and nothing else. A maintainer will
+   contact you privately with a channel for the details.
+
+## What we consider in scope
+
+`qlever-dir` builds a SPARQL endpoint from untrusted files dropped into a
+mounted directory, and its index-build path shells out (`rapper`, `sed`) to
+do it. In scope:
+
+- Anything that lets a crafted filename or file content escape the intended
+  shell/SPARQL context during indexing (see the open injection-shaped issues:
+  [#5](https://github.com/retinue-os/qlever-dir/issues/5),
+  [#6](https://github.com/retinue-os/qlever-dir/issues/6),
+  [#8](https://github.com/retinue-os/qlever-dir/issues/8) — those are filed,
+  triaged, and not what this policy is for; a *new* way to turn one into code
+  execution or data exfiltration rather than a build error is).
+- Anything that lets the watcher or build process read or write outside the
+  mounted data directory.
+
+## Known limitations — please don't report these as vulnerabilities
+
+These are documented, already-filed design gaps, not undiscovered bugs:
+[#4](https://github.com/retinue-os/qlever-dir/issues/4) (watcher failure is
+silent), [#7](https://github.com/retinue-os/qlever-dir/issues/7) (no
+supervision or readiness signal), [#10](https://github.com/retinue-os/qlever-dir/issues/10)
+(watcher misses files in directories created after startup). A report telling
+us these exist tells us nothing new; a report showing one of them is worse
+than described is in scope.