run_smoke: cap each script and kill its process group on expiry

[Jammy2211]

Leg A of the per-script timeout backport. Depends on PyAutoHands#257, which adds the kill_group helper this imports — merge that first (the import is guarded, so an older Hands falls back rather than breaking the gate, but the shared implementation only arrives once #257 lands).

The defect

This runner called subprocess.run(capture_output=True) with no timeout at all. When a script leaves a grandchild holding the inherited stdout pipe, the parent blocks on that pipe forever — even though the script's own work finished and the direct child exited. The gate then sits until the 6-hour GitHub Actions ceiling, reporting nothing since the last completed script. That is autolens_workspace_test#196, and this copy had the same shape.

Reproduced, against this exact file

A two-script suite whose second script spawns a grandchild and exits, with BUILD_SCRIPT_TIMEOUT=4:

	Result
Before	printed ::group::hangs.py and nothing further; still hung when the harness killed it at 30s. The cap was ignored because there was none.
After	[TIMEOUT (4s)] hangs.py — 4.0s, exit 1, zero surviving grandchildren.

What it adopts

The implementation autolens_workspace_test already runs:

• per-script cap via timeout_for(env), so a profile's BUILD_SCRIPT_TIMEOUT override is resolved parent-side where the kill timer lives;
• the child in its own session, so the whole group is killed;
• exit 124 rather than the signal — a timeout means "raise the cap or SLOW-skip it", which -9 would mislabel as an ordinary failure.

timeout_for and kill_group are imported from PyAutoHands build_util, each with a local fallback for a checkout without Hands on PYTHONPATH, so the PR gate and the mega-run share one implementation. The fallback path was verified against a stub build_util exposing timeout_for but no kill_group: still reports TIMEOUT, still zero survivors.

The pre-existing docstring wording in this file is left alone — tidying it is not this PR's job.

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