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