just: Introduce justfile with pre-push hook cmds

As part of migrating from `flake.nix` to a more universal task runner
that can manage git hooks too.

justfile

@@ -0,0 +1,26 @@
+default:
+    @just --list
+
+# Run pre-push checks
+pre-push: lint-rust
+    @echo "✅ pre-push passed"
+
+# Run Clippy lints
+lint-rust: (verify-tool "cargo")
+    @echo "Cargo clippy..."
+    @cargo clippy --workspace --all-targets --all-features -- --deny warnings
+
+# Check if required tools are in PATH.
+[private]
+verify-tool tool package_name="":
+    #!/usr/bin/env bash
+    set -e
+    if ! command -v {{tool}} >/dev/null 2>&1; then
+        PKG="{{package_name}}"
+        if [ -z "$PKG" ]; then
+            PKG="{{tool}}"
+        fi
+        echo "❌ Missing required tool: {{tool}}"
+        echo "💡 Use your systems package manager to install '$PKG'."
+        exit 1
+    fi