docs: #875 follow-up — repo surfaces + reproduction URLs + CHANGELOG

Remaining in-repo surfaces carrying the same retracted or broken
claims as the public pages fixed in the previous two commits.

CONTRIBUTING.md
 - "Palace structure matters ... 34% retrieval improvement" → reframed
   as scoping (same rewording applied to the website equivalents).

benchmarks/BENCHMARKS.md
 - Add a prominent "Important caveat" block at the top of the
   "Comparison vs Published Systems" table explaining that R@5
   (retrieval recall) and QA accuracy are different metrics, with
   citations to Mastra, Mem0, and Supermemory's own published
   methodology pages. Annotate the specific competitor rows whose
   numbers are QA accuracy, not retrieval recall.
 - Annotate the `hybrid v4 + rerank 100%` row to note that the 99.4
   → 100 step was tuned on 3 specific wrong answers (already disclosed
   further down in the doc under "Benchmark Integrity"); the honest
   hybrid figure is held-out 98.4%.
 - Fix the broken clone URL — `aya-thekeeper/mempal` no longer points
   at anything; now `MemPalace/mempalace`.

benchmarks/README.md + benchmarks/HYBRID_MODE.md
 - Same clone-URL fix applied.

CHANGELOG.md
 - Add a ### Documentation entry under [Unreleased] v3.3.0 that names
   #875 and summarises the scope of the rewrite.

benchmarks/HYBRID_MODE.md

@@ -196,9 +196,9 @@ python benchmarks/longmemeval_bench.py data/longmemeval_s_cleaned.json --mode hy
 
 ```bash
 # Setup
-git clone -b ben/benchmarking https://github.com/aya-thekeeper/mempal.git
-cd mempal
-pip install chromadb
+git clone https://github.com/MemPalace/mempalace.git
+cd mempalace
+pip install -e ".[dev]"
 
 # Download data
 mkdir -p /tmp/longmemeval-data