feat: Phase 4 — reusable workflow ("subscribe") + dogfood it

Centralizes the ~90-line consumer stub into a reusable Gitea workflow so a
repo can subscribe to Gadfly with a tiny caller. Feasibility was probe-
verified on this act_runner: workflow_call runs, secrets: inherit
delivers, and a fully-qualified owner/repo/path@ref resolves.

- .gitea/workflows/review-reusable.yml: `on: workflow_call` job holding
  the image pin + all env plumbing. Inputs (models/specialists/provider/
  concurrency/timeouts/allowed_users/…) default to "" so an empty value
  falls back to the image's own default — caller overrides only what it
  wants. Secrets via `secrets: inherit` (optional ones resolve empty).
- adversarial-review.yml: gadfly's own dogfood is now a thin CALLER of the
  reusable (proves it end-to-end; advisory so safe to dogfood).
- examples/reusable.yml: the slim ~8-line consumer stub.
- README / examples/README / CLAUDE.md document the subscribe path.

Caveat: consumers with arbitrary GADFLY_ENDPOINT_<NAME>s still need the
full stub (a reusable workflow can't enumerate dynamic secret names).
YAML validated; Go unchanged (build + test green).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -46,6 +46,9 @@ entrypoint.sh          container brains: trigger gating, PR clone, model loop (t
                        used to live in workflow YAML)
 Dockerfile             multi-stage; private-module creds via BuildKit secrets never reach the final image
 .gitea/workflows/build-image.yml   push main → :latest; tag v* → :<tag>+:latest; PR → build-only
+.gitea/workflows/review-reusable.yml  reusable (workflow_call) review job; consumers subscribe with
+                       an ~8-line caller + `secrets: inherit` (Phase 4). gadfly's own
+                       adversarial-review.yml is a thin caller of it (dogfoods the path).
 examples/              copy-paste consumer stub workflows for different providers
 ```