chore: auto format code on save and add more details to AGENTS.md (#1582)

Adds a default vscode config with generally applicable settings.
Adds more entrypoints to justfile both  for environment setup and to help
agents better verify changes.

.vscode/settings.json

@@ -0,0 +1,10 @@
+{
+    "rust-analyzer.checkOnSave": true,
+    "rust-analyzer.check.command": "clippy",
+    "rust-analyzer.check.extraArgs": ["--all-features", "--tests"],
+    "rust-analyzer.rustfmt.extraArgs": ["--config", "imports_granularity=Item"],
+    "[rust]": {
+        "editor.defaultFormatter": "rust-lang.rust-analyzer",
+        "editor.formatOnSave": true,
+    }
+}