Follow up async task support after rebase

## Context
The async task work from #69 was rebased onto `origin/main` on the `issue-69-async-tasks` branch. The core async support and regression fix are already present in main through commits `ac10957` and `dbf4caa` (and are included in the rebased branch).

## Changes reviewed/restored
- Async `Context.run` / `run_async` behavior and hidden stdout/stderr handling.
- Async task execution context and synchronous pre-task behavior.
- Recursive handling of grouped pre/post tasks during async-task detection and call expansion.
- New async-safety analysis module and tests covering blocking operations, warning/error modes, pre-task checks, and unavailable source.
- Documentation updates and async review findings.

## Follow-up work
The restored review tests currently identify integration work still needed:
- Wire the `--check-async-safety` CLI option into the program and task execution path.
- Resolve grouped pre-task execution/deduplication behavior and the parallel barrier failure.
- Re-run the focused async suite, full nox tests, and pre-commit after integration.

## Validation
- Rebase onto `origin/main` completed; Git skipped the previously applied async regression commit because it is already upstream.
- `git diff --check` passes.
- Focused async tests currently report 7 failures, all in the new async-safety/grouped-task follow-up coverage.
- Full `nox -s tests` reached completion of the test run before the command timeout; pre-commit timed out while installing the detect-secrets environment.