release: 2.5.0 — external security audit landed

- CHANGELOG [2.5.0] documenting the merged security series (#100, #101, #102)
  and wake fixes (#97, #98), with credit to the external contributors
- CHANGELOG [2.4.0] written retroactively: the tag shipped on 2026-06-23
  pointing at "See CHANGELOG.md for details" while the section never existed
- README: version badge and What's New in v2.5
- SECURITY.md: policy now matches actual behaviour — an unanswered private
  channel does not bind a reporter to silence
- version bumps: root 2.5.0, @murmurv2/core 0.4.0, broker-nats 0.3.0,
  mcp-server 0.2.0

Build green, 213/213 tests pass.