fix(zed): switch to language_models.openai_compatible for Agent panel visibility

The Zed Agent panel (new ACP-based UI) only discovers providers registered
under language_models.openai_compatible.<name>, not language_models.openai.
Each named entry appears as its own provider in the LLM Providers list.

- language_models.openai_compatible.harness-sync-gateway holds gateway URL + models
- agent.default_model.provider = 'harness-sync-gateway' (was 'openai')
- Model entries include capabilities {tools,images,parallel_tool_calls}
- Export common.GatewayProviderID so zed.go can reference it
- Fix staticcheck QF1001 (De Morgan's law) in io.go
- Fix struct field alignment in File (80→72 bytes)
- gofmt remaining files

internal/adapters/cagent/cagent.go

@@ -47,6 +47,18 @@ func defaultHome() string {
 // Name returns the harness identifier.
 func (a *Adapter) Name() string { return name }
 
+// Capabilities declares what cagent harness-sync manages.
+func (a *Adapter) Capabilities() adapter.HarnessCapabilities {
+	return adapter.HarnessCapabilities{
+		ManagesProviders:    true,
+		ManagesModels:       true,
+		ManagesMCP:          true,
+		ManagesSkills:       false,
+		ManagesInstructions: true,
+		HasBuiltInSub:       false,
+	}
+}
+
 // Detect returns true when ~/.config/cagent/ exists.
 func (a *Adapter) Detect() bool {
 	_, err := os.Stat(filepath.Join(a.home, ".config", "cagent"))