docs: add SECURITY.md

SECURITY.md

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+# Security
+
+Restorable reads your backup repository and handles your restored data, so
+here's what it does and doesn't promise, and how to tell me if something's
+wrong with it.
+
+## Reporting a vulnerability
+
+Use [GitHub's private reporting](https://github.com/restorable-dev/restorable/security/advisories/new).
+That opens an advisory only the maintainers can see. Please don't file a public
+issue for anything exploitable.
+
+I'll acknowledge within a week. Fixes may take longer, it's a small project, but
+the advisory will say where things stand instead of going quiet.
+
+If you'd rather not use GitHub, open a public issue asking for a contact address
+and leave the details out.
+
+## Supported versions
+
+| Version | Supported |
+| ------- | --------- |
+| 0.1.x   | Yes       |
+| < 0.1.1 | No        |
+
+Anything before v0.1.1 was built with a Go toolchain carrying known stdlib
+vulnerabilities, and those releases have been pulled. There's no auto-update by
+design, so upgrading means reinstalling.
+
+## What the agent actually enforces
+
+Not design intentions, these are structural and have tests:
+
+The restic wrapper takes a closed, unexported subcommand type. Its only values
+are `version`, `snapshots`, `ls`, `restore`, `check`, `dump` and `cat`. There's
+no path to `forget`, `prune` or `backup` because it wouldn't compile.
+
+Sandbox teardown is deferred, so it runs on every exit from a verification run,
+panics included. Containers get force-removed with their volumes on the same
+guarantee.
+
+Free space is checked before a restore starts. An oversized snapshot fails with
+a clear error rather than filling your disk.
+
+The contents of your files never leave your machine. Connected to the hosted
+control plane, the agent sends pass/fail, timings, snapshot IDs, recipe and
+check names, check messages, error strings, and a label for the repository.
+
+Be aware of what those last three carry, because it is more than "metadata"
+suggests. The repository label is your repo string with credentials stripped,
+so it still contains the host and path, and it appears in the title of every
+alert sent to whatever channels you connect. Check messages can name paths from
+inside the snapshot: a checksum mismatch says which file differed. If either is
+more than you want reaching a third-party chat service, run the agent
+standalone and skip the cloud entirely.
+
+Strings go through scrubbing and redaction before they're logged or
+transmitted, including errors from cleanup failures. Credentials embedded in
+repository URLs are stripped before anything can reach a report, and output
+from database checks is stripped of the segments that echo row values, since a
+failed load quotes the data that failed.
+
+The agent polls outbound over HTTPS and opens no inbound ports.
+
+Cloud is optional either way. `restorable test --config agent.yaml` runs the
+whole local loop without an account.
+
+## Known advisories
+
+`govulncheck` flags two against the agent, both through the Docker client
+library: GO-2026-4887 (CVE-2026-34040, high) and GO-2026-4883 (CVE-2026-33997,
+medium).
+
+Neither is exploitable here, and neither is fixable yet.
+
+Both are daemon-side: an AuthZ plugin bypass on oversized request bodies, and an
+off-by-one in plugin privilege validation. This agent is a client. It creates
+containers, execs into them, and removes them over the local Docker socket. It
+doesn't install plugins or use an AuthZ plugin, so it never drives the affected
+paths. If these worry you, your Docker daemon version is what determines your
+exposure, so update Docker.
+
+As for fixing it here: the patches only exist in `github.com/moby/moby/v2`,
+which has published nothing but betas so far. I'm not moving this onto a beta
+dependency over two advisories it doesn't exercise. When moby/moby/v2 goes
+stable the agent migrates and this section goes away.
+
+Dev dependencies of the web control plane may carry their own advisories. Those
+don't ship. `npm audit --omit=dev` on the production tree is the number that
+matters and it should read zero.
+
+## Out of scope
+
+Bugs in restic itself go to [restic](https://github.com/restic/restic). Same for
+the Docker daemon, including the two above.
+
+Anything that needs an attacker who already has write access to your agent
+config or recipe files. Paths inside a recipe are validated and contained, so a
+recipe cannot read outside the restored snapshot, but a recipe also names the
+container images its database and app checks run. Treat one you did not write
+the way you would treat a docker-compose file from the same source.
+
+Raw scanner output with no explanation of how the issue is reachable in this
+codebase.