fix: project model and reasoning_effort into Codex CLI args, stop rewriting .codex/config.toml

- Add model_reasoning_effort field to AgentProfile (domain layer)
- Remove model parameter from codex_config_toml, stop rewriting model in TOML
- Pass model and model_reasoning_effort via Codex CLI -c overrides on every exec
- Update CodexExecSession with new_with_policy_and_overrides factory
- Cover new conversation and resume flows with tests

crates/infrastructure/src/session/factory.rs

@@ -195,7 +195,7 @@ impl AgentSessionFactory for StructuredSessionFactory {
                         None,
                     ),
                 };
-                Arc::new(CodexExecSession::new_with_policy(
+                Arc::new(CodexExecSession::new_with_policy_and_overrides(
                     id,
                     command,
                     cwd,
@@ -203,6 +203,8 @@ impl AgentSessionFactory for StructuredSessionFactory {
                     policy.0,
                     policy.1,
                     policy.2,
+                    profile.model.clone(),
+                    profile.model_reasoning_effort.clone(),
                     env.to_vec(),
                     plan,
                     enforcer,