Mark superseded commits honestly instead of skipped (closes #152)

Gitea cancels an in-flight run when a newer commit lands on the same
branch and records the cancellation as `failure` / "Has been
cancelled". The workflow rewrote that to `skipped`, but Gitea's
Combine() folds `skipped` into `success`, so the combined-status API
returned green for a commit nothing had ever tested. Rewrite it to
`failure` / "Superseded by a newer commit; never tested" instead:
red-but-honest, and never `pending`, which would block the commit
forever.

Re-running the superseded commit would have been better still, but is
not reachable on this Gitea (1.25.4): its API exposes no rerun
endpoint, workflow dispatch takes a ref rather than a SHA, and every
replay would be a full uncached build with no bound on how many pile
up behind a burst of merges.

The step also stops hardcoding its status context: the logic moves into
script/ci-mark-superseded, which derives the context from the workflow
name, job name and event -- the same three values Gitea builds it from
-- and fails loudly when no status on the commit being built carries
that context, so renaming the workflow or the job cannot silently
disable the rewrite. That is item 2 of
#147; item 1 there is
untouched.

Tests drive the script against a fake Gitea covering the cancelled,
laundered-skipped, genuinely-failed, passing and renamed cases, so jq
joins the builder image to run them.

Dockerfile

@@ -32,7 +32,9 @@ FROM golang:1.26.1-bookworm@sha256:4465644228bc2857a954b092167e12aa59c006a349228
 # Depend on lint stage passing
 COPY --from=lint /src/go.sum /dev/null
 
-RUN apt-get update && apt-get install -y --no-install-recommends make && rm -rf /var/lib/apt/lists/*
+# jq is a runtime dependency of script/ci-mark-superseded, which the test
+# suite executes.
+RUN apt-get update && apt-get install -y --no-install-recommends make jq && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /build