v2.14.3 — Repo.transaction/1 changeset error handling docs

Patch release: document the Repo.transaction/1 changeset error-handling
footgun in the ecto-patterns skill (issue #76, proposed by @ndrean).

Added

• ecto-patterns: Repo.transaction/1 changeset error handling — new
  reference section (and an anti-pattern row in the auto-loaded SKILL.md) on the
  bare-match footgun inside Repo.transaction(fn -> ... end): {:ok, _} = Repo.update(cs) raises MatchError on an invalid changeset, which rolls back
  and re-raises (crashing the caller, e.g. a 500) — it does not return
  {:error, %MatchError{}}. Documents the explicit case + Repo.rollback/1
  fix (single step) and the with ... else fix (multi-step), and points to
  Repo.transact/1 / Ecto.Multi as the footgun-free forms. Verified
  empirically on Ecto 3.14. Thanks to @ndrean for the proposal (#76).