gitlab: Implement ChecksByChange with per-job pipeline detail

[ed-irl]

Implements the GitLab side of the per-change checks API: rollup
state from the MR's head pipeline, per-job name/state/url from the
pipeline's job list, and the pipeline's web URL as the click target.

• gateway: Pipeline grows ID and WebURL fields; new Job type
  (name + status + web_url); new PipelineJobsList API method
  wrapping /projects/:id/pipelines/:pipeline_id/jobs.
• rollupFromPipelineStatus collapses GitLab's pipeline status
  enum to the upstream rollup taxonomy. Canceled/manual/skipped
  are user-initiated non-failures and roll up as passed (the
  operator's choice not to run them isn't a failure). Unknown
  statuses default to pending — GitLab adds new statuses over
  time and pending is the safer default.
• pipelineState (the merge handler's path) keeps its 'no
  pipeline -> passed' semantics; ChecksByChange uses ChecksNone
  in that case so the indicator can distinguish.
• ChecksByChange issues MergeRequestGet + PipelineJobsList per
  MR. Jobs are reported in pipeline order (stages first, then
  job order within stages); GitLab's job.status is already
  lowercase so it flows straight through as forge-native state.

Integration fixtures regenerate on the next forgetest -update
pass (real GitLab credentials).

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • forge: Add ChecksByChange and ChecksNone for per-change checks output #1263
    • shamhub: Implement ChecksByChange with full rollup + runs + URL seeding #1264
      • github: Implement ChecksByChange with per-run detail and checks URL #1265
        • gitlab: Implement ChecksByChange with per-job pipeline detail #1266 ◀
          • bitbucket: Implement ChecksByChange with per-status build detail #1267
            • list: Emit per-change forge checks via --cr-checks JSON flag #1268
              • docs: Document forge checks JSON output and add changelog entry #1269

_{Change managed by git-spice.}