feat: improve memory gateway retrieval and publish benchmark page

[cayohollanda]

Summary

• improve Memory Gateway retrieval with scoped qmd collections, heading chunks, symlink-aware pack discovery, and explicit pack-name priority
• add opt-in gateway debug snapshots for local memory-quality diagnosis
• update README/CHANGELOG with anonymized benchmark results and v0.13.0 metadata
• add a simple GitHub Pages landing page and deployment workflow

Validation

• npm run ci
• npm pack --dry-run
• public synthetic benchmark: BENCH_REPEATS=5 BENCH_PROFILES='baseline gateway' BENCH_TIMEOUT=180 bash benchmark/run.sh /tmp/pi-memctx-benchmark-release

Benchmark highlights

• real-world anonymized stress benchmark: 66.9% faster, 80.8% fewer provider tokens, 95.0% fewer tool calls, +25 scored facts
• public synthetic release check: 52.2% faster, 95.2% fewer tool calls, 42.4% fewer visible tokens, +13 scored facts

Privacy

• no private packs, raw benchmark outputs, debug snapshots, local paths, or customer/company data are committed
• real-world benchmark names are anonymized as Pack 1–5