fix(qwen): un-exempt opencode, and keep the Qwen key in a secret

Two findings this round contradicted each other — one asked me to extend the
engine-spec exemption to a bare "opencode", the other said opencode should not
be exempt at all. The code settles it: that engine drives an ollama-cloud model
through the bundled CLI and authenticates with OLLAMA_API_KEY, so it needs
exactly the key the pre-flight checks. Exempting it, which I did last round,
switched the check off for the one engine it could still help. Only claude-code
is exempt now — it carries CLAUDE_CODE_OAUTH_TOKEN and needs no Ollama key —
and opencode/open-code get table rows so both spellings are covered.

The README told operators to embed the Qwen key in a GADFLY_ENDPOINT_* var,
while the workflow that forwards those vars warns in its own comments that vars
are NOT masked. Rather than only rewording the docs, a keyless kimi/qwen
endpoint now falls back to its own QWEN_API_KEY / KIMI_API_KEY — the same
vendor's key, so the no-cross-vendor rule is untouched — which lets the URL live
in a var and the credential in a secret. Break-checked by pointing that fallback
at OPENAI_API_KEY: the leak test catches it.

Smaller: isBuiltinCompatProvider mirrors isOpenAICompatProvider instead of an
inline slices.Contains, with a test that every builtin is also in the compat
list (a builtin missing from it would never reach the branch that protects it);
the preflight.sh rationale is stated once rather than in two comment blocks;
the Go test locates the shell script relative to its own source file; and the
gofmt step takes GOPROXY=off like its neighbours.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

.gitea/workflows/build-image.yml

@@ -135,6 +135,7 @@ jobs:
         env: { GOPROXY: "off" }
         run: go vet ./...
       - name: gofmt
+        env: { GOPROXY: "off" }
         run: test -z "$(gofmt -l .)" || { gofmt -l .; exit 1; }
       - name: go test
         env: { GOPROXY: "off" }