tools: delete ToolOutput, dispatch returns String

ToolOutput was just { text: String } — replaced with plain String.
dispatch() and dispatch_shared() return String directly.
ActiveToolCall handle is (ToolCall, String).
Error results are prefixed with "Error: " by convention.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/mod.rs

@@ -134,20 +134,6 @@ pub struct ToolCallDelta {
     pub function: Option<FunctionCallDelta>,
 }
 
-/// Result of dispatching a tool call.
-pub struct ToolOutput {
-    pub text: String,
-}
-
-impl ToolOutput {
-    pub fn error(e: impl std::fmt::Display) -> Self {
-        Self { text: format!("Error: {}", e) }
-    }
-
-    pub fn text(s: String) -> Self {
-        Self { text: s }
-    }
-}
 
 /// A tool call in flight — metadata for TUI + JoinHandle for
 /// result collection and cancellation.
@@ -157,7 +143,7 @@ pub struct ActiveToolCall {
     pub detail: String,
     pub started: Instant,
     pub background: bool,
-    pub handle: tokio::task::JoinHandle<(ToolCall, ToolOutput)>,
+    pub handle: tokio::task::JoinHandle<(ToolCall, String)>,
 }
 
 /// Truncate output if it exceeds max length, appending a truncation notice.
@@ -170,10 +156,12 @@ pub fn truncate_output(mut s: String, max: usize) -> String {
 }
 
 /// Dispatch a tool call by name through the registry.
+/// Dispatch a tool call by name. Returns the result text,
+/// or an error string prefixed with "Error: ".
 pub async fn dispatch(
     name: &str,
     args: &serde_json::Value,
-) -> ToolOutput {
+) -> String {
     dispatch_with_agent(name, args, None).await
 }
 
@@ -182,16 +170,16 @@ pub async fn dispatch_with_agent(
     name: &str,
     args: &serde_json::Value,
     agent: Option<std::sync::Arc<tokio::sync::Mutex<super::Agent>>>,
-) -> ToolOutput {
+) -> String {
     for tool in tools() {
         if tool.name == name {
             return match (tool.handler)(agent, args.clone()).await {
-                Ok(s) => ToolOutput::text(s),
-                Err(e) => ToolOutput::error(e),
+                Ok(s) => s,
+                Err(e) => format!("Error: {}", e),
             };
         }
     }
-    ToolOutput::error(format!("Unknown tool: {}", name))
+    format!("Error: Unknown tool: {}", name)
 }
 
 /// Dispatch shared tools — used by subconscious agents.
@@ -199,12 +187,12 @@ pub async fn dispatch_shared(
     name: &str,
     args: &serde_json::Value,
     _provenance: Option<&str>,
-) -> Option<ToolOutput> {
+) -> Option<String> {
     for tool in tools() {
         if tool.name == name {
             return Some(match (tool.handler)(None, args.clone()).await {
-                Ok(s) => ToolOutput::text(s),
-                Err(e) => ToolOutput::error(e),
+                Ok(s) => s,
+                Err(e) => format!("Error: {}", e),
             });
         }
     }