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/assistant/mod.rs

@@ -239,7 +239,6 @@ impl TicketAssistantEnvironmentPreparer {
                     &declaration,
                     cwd.as_str(),
                     project.root.as_str(),
-                    profile.model.as_deref(),
                 );
                 self.write_file(&path, rendered.as_bytes()).await?;
                 env.push((home_env.clone(), parent_dir(cwd, target)));
@@ -518,12 +517,8 @@ fn codex_config_toml(
     mcp_declaration: &str,
     run_dir: &str,
     project_root: &str,
-    model: Option<&str>,
 ) -> String {
     let mut text = existing.unwrap_or_default().to_owned();
-    if let Some(model) = model {
-        text = set_top_level_toml_value(&text, "model", model);
-    }
     text = replace_toml_table_block(&text, "mcp_servers.idea", mcp_declaration.trim_end());
     text = replace_toml_table_block(
         &text,
@@ -538,33 +533,6 @@ fn codex_config_toml(
     text
 }
 
-fn set_top_level_toml_value(input: &str, key: &str, value: &str) -> String {
-    let line = format!("{key} = {}", toml_quoted(value));
-    set_top_level_toml_line(input, key, &line)
-}
-
-fn set_top_level_toml_line(input: &str, key: &str, replacement: &str) -> String {
-    let needle = format!("{key} =");
-    let mut out = Vec::new();
-    let mut replaced = false;
-    for line in input.lines() {
-        let trimmed = line.trim_start();
-        if !replaced && trimmed.starts_with(&needle) {
-            out.push(replacement.to_owned());
-            replaced = true;
-        } else {
-            out.push(line.to_owned());
-        }
-    }
-    if !replaced {
-        if !out.is_empty() && !out.first().is_some_and(|line| line.trim().starts_with('[')) {
-            out.push(String::new());
-        }
-        out.insert(0, replacement.to_owned());
-    }
-    out.join("\n")
-}
-
 fn replace_toml_table_block(existing: &str, table: &str, replacement: &str) -> String {
     let header = format!("[{table}]");
     let mut out = Vec::new();
@@ -669,13 +637,7 @@ mod codex_config_toml_tests {
         .to_config_toml();
         let existing = "user_key = \"keep\"\n\n[features.code_mode]\ndirect_only_tool_namespaces = [\"old\"]\n\n[features.preview]\nenabled = true\n\n[user.table]\nvalue = 1\n";
 
-        let rendered = codex_config_toml(
-            Some(existing),
-            &declaration,
-            "/run/assistant",
-            "/proj",
-            None,
-        );
+        let rendered = codex_config_toml(Some(existing), &declaration, "/run/assistant", "/proj");
 
         assert!(
             rendered.contains("user_key = \"keep\""),