Add inline progress for long-running queued tasks

[steve-the-edwards]

Long-running Gradle and benchmark commands currently hold a Model Context Protocol [MCP] tool turn until completion. Developers only see “Run Queued Task,” cannot distinguish quiet progress from a hang, and steering cancels the command.

What changed

• Return short command results normally, but yield a queued/running handle after at most 30 seconds.
• Add bounded task_status updates with queue position, process liveness, incremental inline output, and 30-second heartbeats.
• Keep commands alive when run_task or task_status requests are interrupted; add explicit cancel_task termination.
• Persist terminal results after releasing active queue rows.
• Drain stdout and stderr concurrently to prevent a full pipe from deadlocking the subprocess.

Review focus

Please focus on background task ownership, cancellation races, and persisted result/output-offset lifecycle. A regression test writes 2 MB to stderr before stdout to prove both pipes are drained concurrently.