chore: add Justfile with build, install, test, and fmt commands

Author: urmzd

Justfile

@@ -0,0 +1,15 @@
+# Build all crates
+build:
+    cargo build --workspace
+
+# Install the CLI binary
+install:
+    cargo install --path crates/llmem-cli
+
+# Run all tests
+test:
+    cargo test --workspace
+
+# Format all code
+fmt:
+    cargo fmt --all