feat(engine): add opencode CLI review engine

Add a third review harness alongside the in-process majordomo loop and the
claude-code CLI shell-out: the OpenCode CLI (opencode.ai) driving an ollama-cloud
model, selected by an "opencode/<model>" spec. The goal is to benchmark gadfly's
boutique executus harness against a freely-available agentic harness on the SAME
model (e.g. "ollama-cloud/glm-5.2" vs "opencode/glm-5.2").

OpenCode has no --append-system-prompt flag, so the lens system prompt and the
read-only discipline are delivered through a generated config injected via
OPENCODE_CONFIG_CONTENT: a "gadfly" agent whose prompt is the system prompt with
edit/bash denied at both the global and agent level, plus a "gadfly" ollama-cloud
provider. That env var is the highest-precedence config source in the container,
so a reviewed repo's own opencode.json can't re-enable edits on the reviewer.

Spec forms: "opencode/<model>" (wrapped in the generated provider), the
"open-code/" alias, "opencode/<provider>/<model>" pass-through to OpenCode's own
registry, and bare "opencode". Model ids are taken verbatim so colon-bearing
ollama ids (qwen3-coder:480b-cloud) survive. Auth reuses OLLAMA_CLOUD_API_KEY
(mapped to OLLAMA_API_KEY, referenced as {env:OLLAMA_API_KEY} in config, never a
literal secret). Knobs mirror GADFLY_CLAUDE_*: GADFLY_OPENCODE_BIN/MODEL/BASE_URL/
EXTRA_ARGS. openCodeEnv() forwards OLLAMA_API_KEY (the inverse of claudeEnv) but
still withholds the Gitea/findings/Anthropic secrets.

main.go engine selection is now a switch (claude-code / opencode / majordomo), and
the auto-select path uses a type-check instead of a boolean so a shell-out engine
can never hit the *majordomoEngine assertion. auto-select and delegate_investigation
stay majordomo-only and are skipped for opencode (the CLI does its own legwork).

Dockerfile bundles opencode-ai (npm auto-selects its musl build on alpine) with a
best-effort version check + provider pre-warm that never fails the shared image
build. README/examples/CLAUDE.md/scripts updated per the maintenance rules.

Tests: new opencode_test.go mirrors engine_test.go (spec/model/args/config/env-
filter + stub-CLI runtime tests). Verified end-to-end with a fake opencode CLI:
correct argv, injected config, and consolidated markdown output.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

CLAUDE.md

@@ -34,7 +34,10 @@ verifies each one against the actual code, and posts its findings as a comment.
 cmd/gadfly/            the reviewer binary — pure producer of review markdown (stdout)
   main.go              orchestration: fan specialists out (executus/fanout), each a review pass + recheck
   engine.go            reviewEngine abstraction: executus run.Executor (majordomo agent loop +
-                       compaction/bounding/budget/critic) vs claude-code CLI shell-out
+                       compaction/bounding/budget/critic) vs claude-code / opencode CLI shell-outs
+  opencode.go          the opencode CLI engine: ollama-cloud model through the OpenCode harness
+                       (read-only via a generated OPENCODE_CONFIG_CONTENT agent+provider); for
+                       benchmarking the boutique harness vs a free one on the same model
   executus.go          executus wiring: tool.Registry over the repo tools, the run.Executor build
                        (compact + model context-limit threshold + per-PR budget + wrap-up critic)
   specialists.go       specialist lenses: built-ins, default suite, env + .gadfly.yml resolution