docs: changelog entry for HNSW integration and Python 3.14 build fix

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

CHANGELOG.md

@@ -1,5 +1,31 @@
 # Changelog
 
+## Unreleased
+
+### New Features
+- `clawhdf5-agent`: HNSW now backs the vector stage of `hybrid_search`. The
+  `hnsw` feature is **on by default**, so semantic search uses the approximate
+  `clawhdf5-ann` index instead of a linear cosine scan. The index mirrors the
+  memory cache (node id == cache index) and self-heals — it rebuilds whenever it
+  drifts from the cache length, so no mutation path can desync it. Non-indexable
+  stores (no/zero-dim/mixed embeddings) and dimension-mismatched queries fall
+  back to the exact linear scan. Disable with
+  `--no-default-features --features float16` for exact search.
+- `clawhdf5-ann`: HNSW is now a live, mutable index — added `insert`,
+  `mark_deleted` (soft-delete bitset; deleted nodes are traversed for
+  connectivity but never returned), `compact` (drops deleted vectors and
+  renumbers survivors), and `new` (empty index). Serialization gains a format
+  version tag (`HNSW_FORMAT_VERSION` = 2) and persists the deleted bitset;
+  pre-existing v1 files still load.
+- `clawhdf5-agent`: `hybrid::merge_vector_keyword` exposes the shared
+  normalize-and-fuse step used by both the linear and HNSW vector paths.
+
+### Bug Fixes
+- `clawhdf5-py`: upgrade PyO3 and numpy `0.23` → `0.28` so the bindings build on
+  Python 3.14 (PyO3 0.23 capped at 3.13 and hard-failed `cargo build
+  --workspace`). Updated for the removed `PyObject` alias (`Py<PyAny>`) and the
+  `Python::allow_threads` → `Python::detach` rename.
+
 ## v2.1.0 (2026-04-12)
 
 ### New Features