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/apply/apply_test.go

@@ -26,6 +26,9 @@ func (f *failingAdapter) Render(_ *canonical.Bundle) (*adapter.FileSet, error) {
 func (f *failingAdapter) Import(_ string) (*adapter.ImportResult, error) {
 	return &adapter.ImportResult{}, nil
 }
+func (f *failingAdapter) Capabilities() adapter.HarnessCapabilities {
+	return adapter.HarnessCapabilities{}
+}
 
 type stubAdapter struct {
 	name  string
@@ -44,6 +47,9 @@ func (s *stubAdapter) Render(_ *canonical.Bundle) (*adapter.FileSet, error) {
 	}
 	return fs, nil
 }
+func (s *stubAdapter) Capabilities() adapter.HarnessCapabilities {
+	return adapter.HarnessCapabilities{}
+}
 
 // ---------------------------------------------------------------------------
 // Existing tests