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>

entrypoint.sh

@@ -36,6 +36,9 @@
 #                         e.g. "ollama" local, "openai", "anthropic", "google")
 #   GADFLY_BASE_URL       override backend endpoint (OpenAI/Ollama-compatible servers)
 #   GADFLY_API_KEY        provider key (else provider's standard env: OPENAI_API_KEY, …)
+#   CLAUDE_CODE_OAUTH_TOKEN  auth for the claude-code engine (GADFLY_MODELS entry
+#                         "claude-code"/"claude-code/<model>"); Pro/Max subscription
+#                         token from `claude setup-token`. Else ANTHROPIC_API_KEY.
 #   GADFLY_TRIGGER_PHRASE comment phrase that triggers a re-review (default "@gadfly review")
 #   GADFLY_ALLOWED_USERS  comma-separated usernames allowed to comment-trigger;
 #                         empty => fall back to "is a repo collaborator"