chore: clean repo + professional README rewrite

Removed:
- docs/todo.md (internal dev notes, not for public)
- docs/reports/ (internal validation reports)
- OIDC_SETUP.md (internal CI setup notes, done)
- BENCHMARK_GPU.md (superseded by benchmark_results/ artifacts)
- scripts/train_gnn.py, benchmark_gnn.py, benchmark_neural_hybrid.py,
  train_and_benchmark_large.py, test_full_backprop.py (GNN/experimental paths)
- scripts/benchmark_ler_legacy.py, _bench_large_d.py, build_cp314.bat (stale)
- benchmark_results/*_full.json/.csv (raw verbose dumps, summaries kept)
- benchmark_results/d*_mismatch_audit* (internal debug artifacts)

README.md — complete professional rewrite:
- Correct badge URLs (GuillaumeLessard/qector-decoder)
- pip install one-liner front and center
- Python 3.9-3.13 / Linux / Windows / macOS / GPU info
- Quick start code examples (UnionFind, Blossom, batch, Stim, belief, BP-OSD)
- Full decoder reference table
- Validated claims table linked to benchmark artifacts
- Architecture diagram (Rust core vs Python layer)
- Sinter integration example
- REST API example with security warning
- Honest limitations section
- Full docs index
- CI/CD table
- Commercial model table
- Citation BibTeX
- www.qector.store footer

Author: qectorlab