feat: add --reasoning CLI flag (#314)

This PR adds a new CLI flag: `--reasoning`, which allows users to
customize the reasoning effort level (`low`, `medium`, or `high`) used
by OpenAI's `o` models.
By introducing the `--reasoning` flag, users gain more flexibility when
working with the models. It enables optimization for either speed or
depth of reasoning, depending on specific use cases.
This PR resolves #107

- **Flag**: `--reasoning`
- **Accepted Values**: `low`, `medium`, `high`
- **Default Behavior**: If not specified, the model uses the default
reasoning level.

## Example Usage

```bash
codex --reasoning=low "Write a simple function to calculate factorial"

---------

Co-authored-by: Fouad Matin <169186268+fouad-openai@users.noreply.github.com>
Co-authored-by: yashrwealthy <yash.rastogi@wealthy.in>
Co-authored-by: Thibault Sottiaux <tibo@openai.com>

codex-cli/src/utils/agent/agent-loop.ts

@@ -676,7 +676,7 @@ export class AgentLoop {
           try {
             let reasoning: Reasoning | undefined;
             if (this.model.startsWith("o")) {
-              reasoning = { effort: "high" };
+              reasoning = { effort: this.config.reasoningEffort ?? "high" };
               if (this.model === "o3" || this.model === "o4-mini") {
                 reasoning.summary = "auto";
               }