feat: claude-code reviewer engine (per-lens claude -p shell-out)

Phase 1 of the gadfly-games build. Adds a second review engine alongside
the majordomo agent loop: for each lens, shell out to the Claude Code CLI
(`claude -p`) inside the checked-out repo so it verifies findings with
its OWN read tools, then reuse gadfly's verdict-parse + recheck +
consolidate + emit pipeline unchanged.

- cmd/gadfly/engine.go: new reviewEngine interface with two impls —
  majordomoEngine (wraps the existing runAgent path) and claudeCodeEngine
  (exec `claude -p ... --output-format json`, parse `.result`). main.go's
  runSpecialists/reviewWithSpecialist are now engine-agnostic.
- Select via a model id: `claude-code` (CLI default) or
  `claude-code/<model>` (suffix → --model). Auth inherits from the env:
  Pro/Max via CLAUDE_CODE_OAUTH_TOKEN (no --bare), else ANTHROPIC_API_KEY.
  Read-only by default (--permission-mode plan); tunable via GADFLY_CLAUDE_*.
- auto-select + delegate worker are majordomo-only and are skipped with
  this engine (Claude Code does its own legwork).
- Dockerfile bundles Node + @anthropic-ai/claude-code (larger image).
- Docs: README "Claude Code engine" section + config rows, examples/
  claude-code.yml stub, examples/README + CLAUDE.md updated. Honest note
  that subscription-auth-in-CI is untested here / a ToS gray area.
- Bumps the dogfood image pin to :sha-c3d09d3 so gadfly's own PRs now
  review with the live status board from Phase 3.

New engine_test.go covers spec detection, model derivation, and argv
building (no live CLI call). gofmt clean, go vet quiet, go test -race green.

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

Dockerfile

@@ -24,7 +24,12 @@ RUN --mount=type=cache,target=/go/pkg/mod \
     go build -trimpath -ldflags="-s -w" -o /out/gadfly ./cmd/gadfly
 
 FROM alpine:3.20
-RUN apk add --no-cache bash git curl jq ca-certificates
+RUN apk add --no-cache bash git curl jq ca-certificates nodejs npm
+# Bundle the Claude Code CLI so the `claude-code` review engine works out of the
+# box (GADFLY_MODELS=claude-code or claude-code/<model>). This adds Node + the
+# CLI to the image (notably larger); ollama-only users pay the size but nothing
+# else. Auth is provided at runtime via CLAUDE_CODE_OAUTH_TOKEN / ANTHROPIC_API_KEY.
+RUN npm install -g @anthropic-ai/claude-code && npm cache clean --force
 COPY --from=build /out/gadfly /usr/local/bin/gadfly
 COPY scripts /app/scripts
 COPY entrypoint.sh /entrypoint.sh