Hello Elixir World starter template

Extracted from smolquery's project setup: precommit/ci mix aliases,
credo + ex_slop (checks actually enabled), ex_dna duplication ratchet,
reach architecture policy + smell baseline, dialyzer with cached PLTs,
mix_audit, CI and auto-release workflows, and AGENTS/CONTRIBUTING docs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>