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/domain/src/permission.rs

@@ -851,7 +851,7 @@ pub struct ProjectionContext<'a> {
     /// Absolute isolated run dir of the agent (`.ideai/run/<agent-id>/`).
     pub run_dir: &'a str,
     /// Optional model selected by the agent profile. Orthogonal to permissions:
-    /// projectors may still materialise it even when `eff == None`.
+    /// projectors may still materialise or forward it when appropriate.
     pub model: Option<&'a str>,
 }