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) <[email protected]>

cmd/gadfly/model_test.go

@@ -0,0 +1,25 @@
+package main
+
+import "testing"
+
+func TestBuildSpec(t *testing.T) {
+	tests := []struct {
+		name     string
+		provider string
+		model    string
+		want     string
+	}{
+		{"bare id gets provider prefix", "ollama-cloud", "qwen3-coder:480b-cloud", "ollama-cloud/qwen3-coder:480b-cloud"},
+		{"bare id local ollama", "ollama", "llama3.1", "ollama/llama3.1"},
+		{"already has provider passes through", "ollama-cloud", "openai/gpt-4o", "openai/gpt-4o"},
+		{"slashed model name passes through verbatim", "openai", "openai/meta-llama/Llama-3.1", "openai/meta-llama/Llama-3.1"},
+		{"failover chain passes through", "ollama-cloud", "anthropic/opus-4.8,ollama-cloud/qwen3-coder:480b-cloud", "anthropic/opus-4.8,ollama-cloud/qwen3-coder:480b-cloud"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := buildSpec(tt.provider, tt.model); got != tt.want {
+				t.Errorf("buildSpec(%q, %q) = %q, want %q", tt.provider, tt.model, got, tt.want)
+			}
+		})
+	}
+}