feat: make reasoning effort/summaries configurable

[bolinfest]

Previous to this PR, we always set reasoning when making a request using the Responses API:

codex/codex-rs/core/src/client.rs

Lines 108 to 111 in d7245cb

	reasoning: Some(Reasoning {
	effort: "high",
	summary: Some(Summary::Auto),
	}),

Though if you tried to use the Rust CLI with --model gpt-4.1, this would fail with:

"Unsupported parameter: 'reasoning.effort' is not supported with this model."

We take a cue from the TypeScript CLI, which does a check on the model name:

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

Lines 786 to 789 in d7245cb

	if (this.model.startsWith("o") || this.model.startsWith("codex")) {
	reasoning = { effort: this.config.reasoningEffort ?? "medium" };
	reasoning.summary = "auto";
	}

This PR does a similar check, though also adds support for the following config options:

model_reasoning_effort = "low" | "medium" | "high" | "none"
model_reasoning_summary = "auto" | "concise" | "detailed" | "none"

This way, if you have a model whose name happens to start with "o" (or "codex"?), you can set these to "none" to explicitly disable reasoning, if necessary. (That said, it seems unlikely anyone would use the Responses API with non-OpenAI models, but we provide an escape hatch, anyway.)

This PR also updates both the TUI and codex exec to show reasoning effort and reasoning summaries in the header.