v2.1 — unbreak the action

A maintenance release. No new features and no input changes: v2.1 exists
because v2 had stopped working, in two independent ways.

Fixed

• The action could not start on any repository but this one (#47). GitHub
  runs container actions with --workdir /github/workspace, which overrides the
  image's WORKDIR, so the relative CMD ["python", "src/main.py"] resolved
  against the caller's checked-out repository and failed with
  can't open file '/github/workspace/src/main.py'. The entrypoint is now
  absolute. Reported by @Asthethi; fix from @arjunsuresh in #50.

  This repository's own workflow checked the repository out before running the
  action, which put a src/main.py on the container's working directory and
  made the action appear to work here while it failed everywhere else. That
  checkout has been removed, and CI now builds the image and runs it the way
  GitHub does.

• The OpenAI client could not be constructed. openai was pinned to
  1.30.1 but httpx was not pinned at all. httpx 0.28 removed the proxies
  argument that openai 1.30 passes, so a fresh build failed with
  Client.__init__() got an unexpected keyword argument 'proxies'. All
  dependencies, including transitive ones that affect startup, are now pinned,
  and the Python base image is pinned to a patch version.

Changed

• Default model is now gpt-5.6-luna (was gpt-3.5-turbo, which is
  deprecated and loses API access on 2026-10-23). gpt-5.6-luna is cheaper than
  gpt-3.5-turbo on both input and output tokens, so this does not raise the
  cost of an existing workflow. Pass openai_model to choose a different one.

• Dependencies: openai 1.30.1 → 1.109.1, PyGithub 2.3.0 → 2.10.0,
  requests 2.31.0 → 2.34.2, httpx pinned at 0.28.1.

Documentation

• The workflow example now includes the required permissions block. Missing
  pull-requests: write was a recurring cause of "it runs but posts nothing".
• The configuration reference documented an openai_engine input that the
  action never read. The correct name is openai_model.
• Removed actions/checkout from the example. This action reads the pull
  request over the API and does not need the repository on disk.

Upgrading

None required. v2 now points at this release, and every existing input keeps
its previous meaning. If you pinned openai_model: "gpt-3.5-turbo" explicitly,
change it before 2026-10-23.