feat(core): add configurable log_dir (#10678)

Adds a top-level `log_dir` config key (defaults to `$CODEX_HOME/log`) so
one-off runs can redirect `codex-tui.log` via `-c`, e.g.:

  codex -c log_dir=./.codex-log

Also resolves relative paths in CLI `-c/--config` overrides for
`AbsolutePathBuf` values against the effective cwd (when available).

Tests:
- cargo test -p codex-core

codex-rs/core/src/config_loader/tests.rs

@@ -56,6 +56,30 @@ async fn make_config_for_test(
     .await
 }
 
+#[tokio::test]
+async fn cli_overrides_resolve_relative_paths_against_cwd() -> std::io::Result<()> {
+    let codex_home = tempdir().expect("tempdir");
+    let cwd_dir = tempdir().expect("tempdir");
+    let cwd_path = cwd_dir.path().to_path_buf();
+
+    let config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .cli_overrides(vec![(
+            "log_dir".to_string(),
+            TomlValue::String("run-logs".to_string()),
+        )])
+        .harness_overrides(ConfigOverrides {
+            cwd: Some(cwd_path.clone()),
+            ..Default::default()
+        })
+        .build()
+        .await?;
+
+    let expected = AbsolutePathBuf::resolve_path_against_base("run-logs", cwd_path)?;
+    assert_eq!(config.log_dir, expected.to_path_buf());
+    Ok(())
+}
+
 #[tokio::test]
 async fn returns_config_error_for_invalid_user_config_toml() {
     let tmp = tempdir().expect("tempdir");