Signature verification (gitea hex HMAC-SHA256 over the raw body via
constant-time hmac.Equal; redmine/discourse constant-time shared-secret
headers) with one generic ErrUnverified so rejects give attackers no
oracle. Tolerant normalization of the known Redmine/Discourse/Gitea
payload variants into one Event record (canonical subject ids, actor,
title, repo, sha256 payload digest) plus the DESIGN action mapping
(dispatch_turn / respond_turn / pipeline_step / ignore). Store: JSONL
under state dir, 0600, dedup keyed on provider event id (delivery
header, payload-digest fallback), index rebuilt at startup so replays
across restarts still dedup; torn tail lines skipped, not fatal.
