Serbero v0.2.3

Version 0.2.2 was published to crates.io by mistake, without the startup
check below, and has been yanked. It was never tagged or released on
GitHub. This release contains everything 0.2.2 had, and more.

Changed

• Startup rejects Serbero's own pubkey in [[solvers]] or [[observers]].
  Serbero is registered on Mostro as a solver, but listed in its own config
  it sent itself a copy of every notification, in addition to the other
  configured solvers. If your config lists it, remove that entry before
  upgrading.
• The README explains how solvers read Serbero's messages in Mostrix
  (trusted_dm_senders, the admin key, shared relays).
• Mediation can run as a pilot with the default thresholds of
  docs/judgments.md §3 until a calibration report is published. With
  validated_languages = [], which a pilot keeps, it gathers the facts and
  hands every case to a solver, and never guides a fund action (README
  "Enable mediation", docs/evaluation.md §3.1).