Add a threat model for setup-uv (#923)

This adds a threat model for `setup-uv` so security scanners can use it
as a baseline in terms of what's in-, and out of scope.

The TM covers credential recipients, executable and cache boundaries,
and release authority. It treats checkout-selected interpreters, paths,
virtual environments, symlinks, and helpers as delegated project
authority unless they override an explicit workflow choice or cross an
independent cache, runner, remote, or publication boundary.

SECURITY.md

@@ -0,0 +1,5 @@
+# Security policy
+
+Report suspected vulnerabilities according to [Astral's security policy](https://github.com/astral-sh/.github/blob/main/SECURITY.md).
+
+For this repository's security boundaries and reporting criteria, see the [setup-uv threat model](docs/threat-model.md).