Cut 0.5.0. Headline change is cross_session_synthesis: the reader now receives
an ENTITY SUMMARY consolidating the retrieved facts about a question's named
entities (deduplicated, chronologically ordered, latest dated statement marked
most recent), alongside the deterministic aggregation pre-compute. The container
image now installs from a fully hash-pinned lock with --require-hashes.

- Bump version to 0.5.0 (pyproject, __init__, mcp_server serverInfo, README
  badge, CITATION.cff, .zenodo.json, issue-template placeholder).
- Move the synthesis and container-hardening entries from Unreleased into the
  0.5.0 changelog section (root + docs mirror kept in sync).
- README documents the ENTITY SUMMARY reader hint and its opt-out.

Authored by Anulum Fortis & Arcane Sapience (protoscience@anulum.li)