[codex] Remove smol runtime

[svarlamov]

Summary

• Remove the smol dependency and stale lockfile entries.
• Route synchronous async wait points through a shared Tokio runtime helper.
• Replace bounded smol task/unblock usage with Tokio semaphores and spawn_blocking.
• Update tests and stale docs so repo-wide runtime references are gone.

Validation

• cargo check --all-targets
• task fmt
• task lint
• task test TEST_FILTER=rebase_realworld::test_human_conflict_python_pipeline_mixed_baseline_c3_conflict after one transient full-suite daemon timeout
• task test clean rerun
• rg -n "\\bsmol\\b|smol::" -S Cargo.toml Cargo.lock src tests docs returns no matches

Notes

The initial full task test run had one daemon completion timeout in rebase_realworld::test_human_conflict_python_pipeline_mixed_baseline_c3_conflict; the exact test passed on rerun and the subsequent full suite passed cleanly.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.