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]>

scripts/run.sh

@@ -29,6 +29,13 @@
 # tuning are read straight from the inherited environment — same as the other
 # provider keys (OPENAI_API_KEY, …) — so no extra wiring is needed here.
 #
+# opencode engine: when MODEL is "opencode" or "opencode/<model>" the binary
+# shells out to the bundled `opencode` CLI, driving an ollama-cloud model (for
+# benchmarking against the majordomo path on the same model). Its auth reuses
+# OLLAMA_CLOUD_API_KEY (mapped to OLLAMA_API_KEY below, same as the ollama-cloud
+# path) and GADFLY_OPENCODE_* tuning is read from the inherited environment — so
+# no extra wiring is needed here either.
+#
 # Optional:
 #   MAX_DIFF_CHARS     diff truncation cap for the prompt (default 60000)
 #   GADFLY_STATUS_FILE per-model JSON path for the live status board (set by