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.

codex-rs/justfile

@@ -23,3 +23,9 @@ file-search *args:
 # format code
 fmt:
     cargo fmt -- --config imports_granularity=Item
+
+fix:
+    cargo clippy --fix --all-features --tests --allow-dirty
+
+install:
+    cargo fetch