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/prompt_store/src/prompts.rs

@@ -38,12 +38,12 @@ impl AssistantSystemPromptContext {
 pub struct WorktreeInfoForSystemPrompt {
     pub root_name: String,
     pub abs_path: Arc<Path>,
-    pub rules_file: Option<RulesFile>,
+    pub rules_file: Option<SystemPromptRulesFile>,
 }
 
 #[derive(Serialize)]
-pub struct RulesFile {
-    pub rel_path: Arc<Path>,
+pub struct SystemPromptRulesFile {
+    pub path_in_worktree: Arc<Path>,
     pub abs_path: Arc<Path>,
     pub text: String,
 }