fleetd #550: replace shasum with a portable hash256 helper, add a Linux CI job for the shell suite

jar_id() in redeploy-fleetd.sh called shasum directly, which does not exist on GNU coreutils
Linux (Debian/Ubuntu/etc.) — there it silently reported an existing jar as "absent" with exit 0,
because the missing command made `cut` succeed on empty input and pipefail's failure was then
swallowed by the `|| echo "absent"` fallback. The shell test suite hit the same tool at
test-redeploy-fleetd.sh:298-299 and died at exit 127 with zero FAIL lines printed — the same shape
as a clean pass on the one channel anyone would check.

Adds one hash256() helper (prefer sha256sum, fall back to shasum -a 256, same idiom already used
in probe-member-credentials.sh) and points jar_id and the test suite's own reference hash at it.
jar_id now has three distinct answers instead of two: absent, a hash, or "unhashable" when neither
hasher is on PATH — "absent" is never used for a file that exists.

Adds a CI job (shell-tests) that runs scripts/test-redeploy-fleetd.sh on ubuntu-latest, gated on
the step's own exit code rather than a FAIL-line count, since a suite that dies before running is
exactly what a green run also looks like by that count.

New tests: test_jar_id_reports_unhashable_when_no_hasher_on_path (stubbed PATH with neither
hasher) and test_no_unguarded_macos_only_hasher_calls (a shape check across every script under
scripts/, not named lines — #545 already showed this idiom spreading from two sites to six).

.gitea/workflows/ci.yml

@@ -58,6 +58,23 @@ jobs:
           done
           exit 0
 
+  # fleetd #550 — nothing ran scripts/test-redeploy-fleetd.sh in CI before this, on any platform,
+  # so it had run only on macOS by hand and two Linux-only bugs (this issue's items 1 and 2)
+  # survived undetected: shasum is a macOS-only tool (it ships with Perl; GNU coreutils, i.e. every
+  # mainstream Linux distro including this runner's ubuntu-latest, does not have it and ships
+  # sha256sum instead). The gate here is the step's own exit code, nothing else: a `run:` step in
+  # Gitea/GitHub Actions already fails the job on a non-zero exit with no extra scripting needed,
+  # so this deliberately does NOT grep the output for a `FAIL:` count. That is the #550 item-2
+  # lesson one level up — a suite that dies before it runs a single test prints zero FAIL lines,
+  # which is exactly what a clean pass also prints, so counting FAIL lines can never be the gate.
+  shell-tests:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: redeploy-fleetd.sh shell suite
+        run: bash scripts/test-redeploy-fleetd.sh
+
   # CB-521 — actually run the AMQP contract test in CI, against a REAL broker. The broker is a
   # RabbitMQ SERVICE CONTAINER, not Testcontainers-with-Docker: the runner image has no Docker, so
   # AmqpReplyInboxContractTest reads AMQP_URI (set below to the service's network alias) and binds