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/opencode/opencode.go

@@ -41,6 +41,18 @@ func defaultHome() string {
 // Name returns the harness identifier.
 func (a *Adapter) Name() string { return name }
 
+// Capabilities declares what opencode harness-sync manages.
+func (a *Adapter) Capabilities() adapter.HarnessCapabilities {
+	return adapter.HarnessCapabilities{
+		ManagesProviders:    true,
+		ManagesModels:       true,
+		ManagesMCP:          true,
+		ManagesSkills:       true,
+		ManagesInstructions: true,
+		HasBuiltInSub:       false,
+	}
+}
+
 // Detect returns true when ~/.config/opencode exists.
 func (a *Adapter) Detect() bool {
 	_, err := os.Stat(filepath.Join(a.home, ".config", "opencode"))
@@ -55,6 +67,13 @@ func (a *Adapter) Render(b *canonical.Bundle) (*adapter.FileSet, error) {
 	fs := adapter.NewFileSet()
 	base := filepath.Join(a.home, ".config", "opencode")
 
+	// opencode reads skills from ~/.config/opencode/skills/<name>/SKILL.md
+	fs.Add(adapter.File{
+		Dest:          filepath.Join(base, "skills"),
+		Kind:          adapter.SymlinkDir,
+		SymlinkTarget: filepath.Join(b.Root, "skills"),
+	})
+
 	cfgPath := filepath.Join(base, "opencode.jsonc")
 	existing, _ := os.ReadFile(cfgPath)
 	overlay := map[string]any{