Repo hygiene: drop master remnants, GHCR-prebuilt runner image, ignore local artifacts

[GiggleLiu]

What

• Delete root index.html — it redirects to leaderboard/, which no longer exists on main (Pages deploys site/ via publish-on-merge.yml). Dead artifact at the repo root.
• Drop master from CI triggers — the branch is gone (it was strictly behind main, 0 commits ahead); ci.yml and publish-on-merge.yml still listed it.
• Prebuilt runner image via GHCR — new runner-image.yml builds problem-reductions-runner:<PR_REF> (tag read from the Makefile) on changes to runner inputs and pushes to GHCR. make runner-pull pulls + retags it. This removes the ~30–60 min local Rust compile (CARGO_BUILD_JOBS=1) from the documented run path.
• README — make runner-pull documented as the fast path for the Model API backend; make runner-build remains the fallback.
• .gitignore — ignore .pytest_cache/, .venv/, .claude/, /docs/, /reports/ (local tooling state / personal artifacts that were showing up as untracked clutter).

Done separately (not in this diff)

• Closed landed issues: M4.1 — [Tracking] Harden and release the benchmark #8 (M4.1 tracker), M4.1a — README and contributor guide #10, M4.1b — Calibration tests in CI #11, M4.1c — Session reproducibility archive #12, M4.1d — Observability: per-session cost and failure report #13, Solve-allowed verification: one round-trip check (solve_mismatch) that subsumes all violation types #16 — each with a comment citing the covering code on main. Solve-allowed verification: one round-trip check (solve_mismatch) that subsumes all violation types #16 (solve-allowed verification) is implemented in benchmark/verify.py with stubbed-solver tests; the stale solve-mismatch-verification branch re-implements it against the pre-main codebase and can be abandoned.
• Deleted the stale master branch ref (it no longer existed on the remote) and pointed origin/HEAD at main.
• Updated origin to the new repo location (CodingThrust/problem-reductions-benchmark).

Notes

• One-time step after merge: the first runner-image.yml run creates the GHCR package as private — mark it public (Packages → problem-reductions-runner → Package settings) so make runner-pull works without a PAT.
• The workflow triggers on pushes to main touching runner inputs, so it can't be exercised by this PR itself; YAML validated locally, and the PR_REF extraction (grep/sed) was tested on macOS and uses POSIX classes for the Ubuntu runner.
• Verified locally: make help renders with runner-pull, test_docs.py 8/8 pass.