bxcore, brokerd, loopd: an invalid-arguments denial says why

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/loopd/src/tools.rs

@@ -378,6 +378,19 @@ pub fn denial_text(reason: bxcore::DenyReason) -> &'static str {
     }
 }
 
+/// What a denial reaches the model as: the reason's sentence, and the broker's detail, if it sent
+/// one, after a colon (`Denied: the arguments are not valid for this tool: <detail>.`).
+pub fn denial_message(reason: bxcore::DenyReason, detail: Option<&str>) -> String {
+    let sentence = denial_text(reason);
+    match detail {
+        None => sentence.to_string(),
+        Some(detail) => {
+            let stem = sentence.strip_suffix('.').unwrap_or(sentence);
+            format!("{stem}: {detail}.")
+        }
+    }
+}
+
 pub struct FakeTools {
     calls: Mutex<Vec<bxcore::ToolRequest>>,
 }
@@ -432,6 +445,7 @@ impl ToolPort for FakeTools {
             },
             _ => bxcore::ToolResponse::Denied {
                 reason: bxcore::DenyReason::NoGrant,
+                detail: None,
             },
         }
     }