core: add a feature to disable the shell tool

[nornagon-openai]

--disable shell_tool disables the built-in shell tool. This is useful for MCP-only operation.

[chatgpt-codex-connector]

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[nornagon-openai]

@codex fix

error[E0382]: borrow of moved value: `shell_handler`
   --> core/src/tools/spec.rs:999:39
    |
961 |     let shell_handler = Arc::new(ShellHandler);
    |         ------------- move occurs because `shell_handler` has type `std::sync::Arc<tools::handlers::shell::ShellHandler>`, which does not implement the `Copy` trait
...
995 |         builder.register_handler("local_shell", shell_handler);
    |                                                 ------------- value moved here
...
999 |     builder.register_handler("shell", shell_handler.clone());
    |                                       ^^^^^^^^^^^^^ value borrowed here after move
    |
note: consider changing this parameter type in method `register_handler` to borrow instead if owning the value isn't necessary
   --> core/src/tools/registry.rs:187:74
    |
187 |     pub fn register_handler(&mut self, name: impl Into<String>, handler: Arc<dyn ToolHandler>) {
    |            ---------------- in this method                               ^^^^^^^^^^^^^^^^^^^^ this parameter takes ownership of the value
help: clone the value to increment its reference count
    |
995 |         builder.register_handler("local_shell", shell_handler.clone());
    |                                                              ++++++++