agent: Support AGENT.md and AGENTS.md as rules file names (#31998)

These started to be used more recently, so we should also support them.

Release Notes:

- agent: Added support for `AGENT.md` and `AGENTS.md` as rules file
names.

crates/agent/src/thread_store.rs

@@ -70,13 +70,15 @@ impl Column for DataType {
     }
 }
 
-const RULES_FILE_NAMES: [&'static str; 6] = [
+const RULES_FILE_NAMES: [&'static str; 8] = [
     ".rules",
     ".cursorrules",
     ".windsurfrules",
     ".clinerules",
     ".github/copilot-instructions.md",
     "CLAUDE.md",
+    "AGENT.md",
+    "AGENTS.md",
 ];
 
 pub fn init(cx: &mut App) {