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/adapter/common/blocks.go

@@ -10,6 +10,9 @@ import (
 
 const gatewayProviderID = "harness-sync-gateway"
 
+// GatewayProviderID is the canonical provider key used across all adapters.
+const GatewayProviderID = gatewayProviderID
+
 // ProviderEntry is a generic provider record. Adapters may massage further.
 type ProviderEntry = map[string]any
 
@@ -161,7 +164,6 @@ func ProvidersAsMap(p *canonical.Profile) map[string]any {
 	return out
 }
 
-
 // KiloModelString returns the "providerID/modelID" string used by kilo/opencode.
 func KiloModelString(p *canonical.Profile) string {
 	if p.Gateway.DefaultModel == "" {