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/adapter_test.go

@@ -34,6 +34,7 @@ func (f *fakeAdapter) Name() string                                 { return f.n
 func (f *fakeAdapter) Detect() bool                                 { return f.detect }
 func (f *fakeAdapter) Render(_ *canonical.Bundle) (*FileSet, error) { return NewFileSet(), nil }
 func (f *fakeAdapter) Import(_ string) (*ImportResult, error)       { return &ImportResult{}, nil }
+func (f *fakeAdapter) Capabilities() HarnessCapabilities            { return HarnessCapabilities{} }
 
 func TestRegistry(t *testing.T) {
 	r := NewRegistry()