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_rules/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "agent_rules"
+version = "0.1.0"
+edition.workspace = true
+publish.workspace = true
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/agent_rules.rs"
+doctest = false
+
+[dependencies]
+anyhow.workspace = true
+fs.workspace = true
+gpui.workspace = true
+prompt_store.workspace = true
+util.workspace = true
+worktree.workspace = true
+
+[dev-dependencies]
+indoc.workspace = true

crates/agent_rules/LICENSE-GPL

@@ -0,0 +1 @@
+../../LICENSE-GPL

crates/agent_rules/src/agent_rules.rs

@@ -0,0 +1,51 @@
+use std::sync::Arc;
+
+use anyhow::{Context as _, Result};
+use fs::Fs;
+use gpui::{App, AppContext, Task};
+use prompt_store::SystemPromptRulesFile;
+use util::maybe;
+use worktree::Worktree;
+
+const RULES_FILE_NAMES: [&'static str; 6] = [
+    ".rules",
+    ".cursorrules",
+    ".windsurfrules",
+    ".clinerules",
+    ".github/copilot-instructions.md",
+    "CLAUDE.md",
+];
+
+pub fn load_worktree_rules_file(
+    fs: Arc<dyn Fs>,
+    worktree: &Worktree,
+    cx: &App,
+) -> Option<Task<Result<SystemPromptRulesFile>>> {
+    let selected_rules_file = RULES_FILE_NAMES
+        .into_iter()
+        .filter_map(|name| {
+            worktree
+                .entry_for_path(name)
+                .filter(|entry| entry.is_file())
+                .map(|entry| (entry.path.clone(), worktree.absolutize(&entry.path)))
+        })
+        .next();
+
+    // Note that Cline supports `.clinerules` being a directory, but that is not currently
+    // supported. This doesn't seem to occur often in GitHub repositories.
+    selected_rules_file.map(|(path_in_worktree, abs_path)| {
+        let fs = fs.clone();
+        cx.background_spawn(maybe!(async move {
+            let abs_path = abs_path?;
+            let text = fs
+                .load(&abs_path)
+                .await
+                .with_context(|| format!("Failed to load assistant rules file {:?}", abs_path))?;
+            anyhow::Ok(SystemPromptRulesFile {
+                path_in_worktree,
+                abs_path: abs_path.into(),
+                text: text.trim().to_string(),
+            })
+        }))
+    })
+}