tools: add Tool registry with handlers

Tool struct wraps ToolDef + async handler function. tools() returns
the complete registry — single source of truth for definitions and
dispatch.

Handler signature: fn(Option<Arc<Mutex<Agent>>>, Value) -> BoxFuture<Result<String>>

All tools registered: file ops, bash, web, vision, memory (15 tools),
channels (4 tools), control (3 tools). Working stack removed from
registry (will be replaced).

Old dispatch functions remain for now — next step is to route
dispatch through the registry.

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

src/agent/tools/vision.rs

@@ -48,6 +48,12 @@ pub(super) fn definition() -> ToolDef {
     )
 }
 
+/// Text-only version for the Tool registry.
+pub fn view_image_text(args: &serde_json::Value) -> anyhow::Result<String> {
+    let output = view_image(args)?;
+    Ok(output.text)
+}
+
 /// View an image file or capture a tmux pane.
 pub(super) fn view_image(args: &serde_json::Value) -> Result<ToolOutput> {
     let a: Args = serde_json::from_value(args.clone())