feat(memory): HybridRetriever + progressive enhancement flags ship as v0.1.248

Cumulative Steps 2-7 rollout:

- Step 3 (GREEN-soft): HybridRetriever = BM25 + dense RRF + Cohere rerank blend,
  shipping as the primary retrieval upgrade on top of dense-only baseline.
  Phase B N=500 at gpt-4o measured 73.2% unweighted on LongMemEval-S.
- Step 2 (RED): SessionRetriever (rerank-skip root cause)
- Step 4 (RED): HyDE query rewriting (hypothesis replaces concrete tokens)
- Step 5 (RED): FactSupersession LLM post-filter (over-drops)
- Step 6 (RED): Split-on-Ambiguous rerank refinement (monotonic on wrong axis)
- Step 7 (RED): Observer/Reflector pipeline wired as additive overlay (dilutes retrieval pool)

All experimental features ship opt-in; HybridRetriever is the only production-grade addition.
Bench adapter (agentos-bench) measures every feature via matched-ablation + bootstrap CI + kill-criterion methodology. Umbrella findings and per-step docs in packages/agentos-bench/docs/.

Author: jddunn