v0.15.0

Added

• Incremental re-review (#57). On a push to an open PR, review.yml
  passes --incremental: gatehouse reviews only the commits since its last
  complete review, fetched through the compare API with nothing checked
  out. It falls back to the whole PR for a first review, a force-push or
  rebase, when the last review had an unfinished agent, or when the compare
  fails. Files the PR does not touch are cut from the range, so merging the
  base branch in does not review the base's changes. The review summary
  states the scope.
• Serving model and usage per agent (#58). Each agent logs
  agent=… model=… prompt=… completion=… reasoning=… cost=$… fallback=yes|no
  to stderr, followed by a total line. In CI the same table goes to the job's
  step summary, and --usage-json PATH writes it as JSON.
• Fallback is visible. When a fallback model served any agent, the posted
  review says so, e.g. "3 of 8 agents served by google/gemini-3.1-flash-lite".

Changed

• Findings on a line outside the PR diff are not posted, and the summary
  counts them. GitHub rejects a whole review over one such comment.
• The posted review is pinned to the head commit that was reviewed, and a
  review in which every agent finished carries a hidden
  <!-- gatehouse review complete --> marker; incremental review starts
  from the newest one.
• call_model returns a Completion (text, requested and serving model,
  usage) instead of the bare text.