test(ci): exercise the dsh identity probe with timeout missing (bash 3.2)

[Ark0N]

Closes the loop on #382, which was fixed in 1.28.2 with nothing guarding the fix.

Why the existing job does not cover it

The install.sh runs on bash 3.2 step added with #380 cannot reach dsh_banner_probe at all:

• the bash:3.2 image ships timeout at /usr/bin/timeout, so the optional-prefix array is never empty and the branch that aborts on macOS is never taken;
• there is no dsh binary anywhere in the container, so _cli_candidate_ok never dispatches to the probe in the first place.

And the failure is a runtime abort under set -u, so bash -n cannot see it either. That combination is why the reporter had to find this by reading the source rather than by running anything, and it would let the same shape come back unnoticed.

What this adds

A second bash:3.2 container in the same step that calls the probe directly with timeout hidden behind a narrowed PATH:

• refuses to pass if timeout is still reachable, so the guard cannot quietly stop exercising its own branch;
• asserts a real DeepSeek Harness banner is accepted;
• asserts Debian's unrelated dsh (dancer's shell) is refused, which covers the identity half of the probe.

No production code changes, no changeset: this adds coverage for behaviour that already shipped.

Verified

Ran the step's real run: block, extracted from the YAML so the quoting is the quoting CI will use:

bash 3.2.57(1)-release: 10 CLIs, 0 found
bash 3.2.57(1)-release: skipping the AI CLI install menu continues
bash 3.2.57(1)-release: dsh identity probe survives a missing timeout

Then reverted install.sh to the pre-fix expansion and ran it again, which fails with the exact error from the issue:

/w/install.sh: line 446: runner[@]: unbound variable
BLOCK_EXIT=1

[Ark0N]

Merged, and shipping in 1.29.0: https://github.com/Ark0N/Codeman/releases/tag/codeman@1.29.0. The job log shows the step really runs on the bash 3.2 image (dsh identity probe survives a missing timeout), which is the whole point of guarding a set -u abort that bash -n cannot see.