docs: add Z3 system library to prerequisites

The prover crate now links against system libz3 instead of compiling
from source. Document the install steps for macOS, Ubuntu, and Fedora,
and note the bundled-z3 feature flag as a fallback.

Signed-off-by: Alexander Watson <zredlined@gmail.com>
Made-with: Cursor

Author: zredlined

CONTRIBUTING.md

@@ -119,6 +119,28 @@ Project requirements:
 - Rust 1.88+
 - Python 3.12+
 - Docker (running)
+- Z3 solver library (for the policy prover crate)
+
+### Z3 installation
+
+The `openshell-prover` crate links against the system Z3 library via pkg-config.
+
+```bash
+# macOS
+brew install z3
+
+# Ubuntu / Debian
+sudo apt install libz3-dev
+
+# Fedora
+sudo dnf install z3-devel
+```
+
+If you prefer not to install Z3 system-wide, you can compile it from source as a one-time step:
+
+```bash
+cargo build -p openshell-prover --features bundled-z3
+```
 
 ## Getting Started