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/mod.rs

@@ -144,7 +144,15 @@ pub async fn load_config_layers_state(
     let cli_overrides_layer = if cli_overrides.is_empty() {
         None
     } else {
-        Some(overrides::build_cli_overrides_layer(cli_overrides))
+        let cli_overrides_layer = overrides::build_cli_overrides_layer(cli_overrides);
+        let base_dir = cwd
+            .as_ref()
+            .map(AbsolutePathBuf::as_path)
+            .unwrap_or(codex_home);
+        Some(resolve_relative_paths_in_config_toml(
+            cli_overrides_layer,
+            base_dir,
+        )?)
     };
 
     // Include an entry for the "system" config folder, loading its config.toml,