feat: multi-provider model support via majordomo (local Ollama, OpenAI-compatible, etc.)

Replace the hardcoded ollama.Cloud binding with majordomo's provider registry,
so Gadfly can target any backend majordomo supports without code changes.

- cmd/gadfly/model.go: resolveModel() — GADFLY_PROVIDER (default ollama-cloud)
  prefixes bare model ids; GADFLY_MODEL may be a full provider/model spec, alias,
  or failover chain (verbatim). GADFLY_BASE_URL constructs openai/ollama/anthropic/
  google directly at a custom endpoint (OpenAI-compatible + local/remote Ollama).
  GADFLY_API_KEY else the provider's standard env var. + buildSpec unit tests.
- run.sh: provider-aware key gate (local Ollama needs none); maps OLLAMA_CLOUD_API_KEY
  -> OLLAMA_API_KEY; provider/base-url/key inherited by the binary. Gadfly-branded comment.
- entrypoint.sh: GADFLY_MODELS alias for OLLAMA_REVIEW_MODELS; provider passthrough.
- examples + README: Models & providers section. Upfront: only the Ollama paths
  (local + OpenAI-compatible-against-Ollama) are tested; OpenAI/Anthropic/Google
  are wired via majordomo but UNTESTED (no spend).

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

go.mod

@@ -3,3 +3,23 @@ module gitea.stevedudenhoeffer.com/steve/gadfly
 go 1.26.2
 
 require gitea.stevedudenhoeffer.com/steve/majordomo v0.0.0-20260610113006-0147a79d187b
+
+require (
+	cloud.google.com/go v0.116.0 // indirect
+	cloud.google.com/go/auth v0.9.3 // indirect
+	cloud.google.com/go/compute/metadata v0.5.0 // indirect
+	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
+	github.com/google/go-cmp v0.6.0 // indirect
+	github.com/google/s2a-go v0.1.8 // indirect
+	github.com/googleapis/enterprise-certificate-proxy v0.3.4 // indirect
+	github.com/gorilla/websocket v1.5.3 // indirect
+	go.opencensus.io v0.24.0 // indirect
+	golang.org/x/crypto v0.36.0 // indirect
+	golang.org/x/net v0.38.0 // indirect
+	golang.org/x/sys v0.31.0 // indirect
+	golang.org/x/text v0.23.0 // indirect
+	google.golang.org/genai v1.59.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20240903143218-8af14fe29dc1 // indirect
+	google.golang.org/grpc v1.66.2 // indirect
+	google.golang.org/protobuf v1.34.2 // indirect
+)