feat: map harness session id to Codex threads

[mikehostetler]

Summary

• default Codex thread_id from RunRequest.session_id
• preserve existing Codex metadata/runtime override precedence
• update jido_harness lock to include RunRequest.session_id
• add regression coverage for harness session_id and metadata override behavior

Refs #19.

Verification

• mix test test/jido/codex/adapter_test.exs test/jido/codex/options_test.exs
• mix test
• mix format --check-formatted
• mix compile --warnings-as-errors
• git diff --check

Note: the stdin race tracked by #19 still depends on upstream external_runtime_transport; this PR lands the harness-layer session bridge while keeping the upstream blocker visible.