feat: multi-provider support — Ollama Cloud + OpenCode Go

- Account.Provider field (ollama-cloud | opencode-go), backward compatible
- Model-based routing: common models served by combined pool, unique models
  routed to their provider only
- /go/v1/* path forces OpenCode Go provider (prefix stripped upstream)
- Merged /v1/models endpoint returns union of both catalogs (44 models)
- Failover: 429/402 → cooldown + failover; 5xx → retry without cooldown
- CLI: accounts add --provider flag, list shows provider column
- Body buffering: request body buffered (8 MiB cap) for failover replay
- opencode integration: unified provider 'oc' with all merged models
- 64 tests pass (unit + integration)
- Verified: glm-5 → ollama, mimo-v2.5 → go, gpt-oss:20b → ollama

internal/config/accounts_test.go

@@ -77,6 +77,60 @@ func TestValidateName(t *testing.T) {
 	}
 }
 
+func TestProviderType_Constants(t *testing.T) {
+	if ProviderOllamaCloud != "ollama-cloud" {
+		t.Errorf("ProviderOllamaCloud = %q", ProviderOllamaCloud)
+	}
+	if ProviderOpenCodeGo != "opencode-go" {
+		t.Errorf("ProviderOpenCodeGo = %q", ProviderOpenCodeGo)
+	}
+	if !ValidProvider(ProviderOllamaCloud) || !ValidProvider(ProviderOpenCodeGo) {
+		t.Error("ValidProvider returned false for known providers")
+	}
+	if ValidProvider(ProviderType("bogus")) {
+		t.Error("ValidProvider returned true for bogus provider")
+	}
+}
+
+func TestDefaultProviderBaseURL(t *testing.T) {
+	if DefaultProviderBaseURL(ProviderOllamaCloud) != "https://ollama.com" {
+		t.Errorf("ollama base url = %q", DefaultProviderBaseURL(ProviderOllamaCloud))
+	}
+	if DefaultProviderBaseURL(ProviderOpenCodeGo) != "https://opencode.ai/zen/go" {
+		t.Errorf("go base url = %q", DefaultProviderBaseURL(ProviderOpenCodeGo))
+	}
+}
+
+// TestLoadAccounts_BackwardCompatProvider verifies that accounts.json entries
+// without a "provider" field are normalised to ollama-cloud on load.
+func TestLoadAccounts_BackwardCompatProvider(t *testing.T) {
+	p := withTempAccountsPath(t)
+	// Write an old-style file with no provider field.
+	old := `{
+	  "base_url": "https://ollama.com",
+	  "accounts": [
+	    {"id":"abc","name":"legacy","api_key":"sk-xyz","created":"2026-06-19T10:00:00Z"},
+	    {"id":"def","name":"go1","provider":"opencode-go","api_key":"sk-go","created":"2026-06-19T10:00:00Z"}
+	  ]
+	}`
+	if err := os.WriteFile(p, []byte(old), 0o600); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	af, err := LoadAccounts()
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	if len(af.Accounts) != 2 {
+		t.Fatalf("len = %d", len(af.Accounts))
+	}
+	if af.Accounts[0].Name != "legacy" || af.Accounts[0].Provider != ProviderOllamaCloud {
+		t.Errorf("legacy account provider = %q, want ollama-cloud", af.Accounts[0].Provider)
+	}
+	if af.Accounts[1].Provider != ProviderOpenCodeGo {
+		t.Errorf("go account provider = %q, want opencode-go", af.Accounts[1].Provider)
+	}
+}
+
 func TestLoadAccounts_MissingFile(t *testing.T) {
 	withTempAccountsPath(t)
 	af, err := LoadAccounts()