Add ast_grep tool: structural code search via ast-grep

AST-level pattern matching — find code by structure, not text.
e.g. find all `if let Some($X) = $Y { $$$BODY }` patterns.
Supports C, Rust, Python, JS/TS, Go, and 20+ languages.

Gracefully errors if sg binary isn't installed.

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

src/agent/tools/mod.rs

@@ -5,6 +5,7 @@
 // working_stack) and delegates everything else to thought::dispatch.
 
 // Core tools
+mod ast_grep;
 mod bash;
 pub mod channels;
 mod edit;
@@ -160,7 +161,7 @@ pub fn tools() -> Vec<Tool> {
     let mut all = vec![
         read::tool(), write::tool(), edit::tool(),
         grep::tool(), glob::tool(), bash::tool(),
-        vision::tool(),
+        ast_grep::tool(), vision::tool(),
     ];
     all.extend(web::tools());
     all.extend(memory::memory_tools());