WIP: Fix mind/, dmn, UI layer — 35 errors remaining

mind/mod.rs and mind/dmn.rs fully migrated to AST types.
user/context.rs, user/widgets.rs, user/chat.rs partially migrated.
Killed working_stack tool, tokenize_conv_entry, context_old.rs.

Remaining: learn.rs (22), oneshot.rs (5), subconscious.rs (3),
chat.rs (3), widgets.rs (1), context.rs (1).

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

src/agent/tools/mod.rs

@@ -123,7 +123,6 @@ pub fn tools() -> Vec<Tool> {
         read::tool(), write::tool(), edit::tool(),
         grep::tool(), glob::tool(), bash::tool(),
         vision::tool(),
-        working_stack::tool(),
     ];
     all.extend(web::tools());
     all.extend(memory::memory_tools());