Pull out plain rules file loading code into a new agent_rules crate (#28383)

Also renames for rules file templated into the system prompt

Release Notes:

- N/A

crates/agent/Cargo.toml

@@ -19,6 +19,7 @@ test-support = [
 ]
 
 [dependencies]
+agent_rules.workspace = true
 anyhow.workspace = true
 assistant_context_editor.workspace = true
 assistant_settings.workspace = true