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/crush/crush.go

@@ -41,6 +41,18 @@ func defaultHome() string {
 // Name returns the harness identifier.
 func (a *Adapter) Name() string { return name }
 
+// Capabilities declares what crush harness-sync manages.
+func (a *Adapter) Capabilities() adapter.HarnessCapabilities {
+	return adapter.HarnessCapabilities{
+		ManagesProviders:    true,
+		ManagesModels:       true,
+		ManagesMCP:          true,
+		ManagesSkills:       true,
+		ManagesInstructions: false,
+		HasBuiltInSub:       false,
+	}
+}
+
 // Detect returns true when ~/.config/crush exists.
 func (a *Adapter) Detect() bool {
 	_, err := os.Stat(filepath.Join(a.home, ".config", "crush"))
@@ -69,6 +81,7 @@ func (a *Adapter) Render(b *canonical.Bundle) (*adapter.FileSet, error) {
 	if roleModels := common.CrushRoleModels(&b.Profile); len(roleModels) > 0 {
 		overlay["models"] = roleModels
 	}
+	overlay["default_model"] = b.Profile.Gateway.DefaultModel
 	if mcp := common.BuildMCPMapStyled(&b.MCP, common.MCPCrushStyle); len(mcp) > 0 {
 		overlay["mcp"] = mcp
 	}

internal/adapters/crush/crush_test.go

@@ -53,8 +53,8 @@ func TestCrushRenderProducesExpectedTargets(t *testing.T) {
 	assert.Contains(t, models, "large")
 	assert.Contains(t, models, "small")
 	assert.Contains(t, models, "title")
-	// Must NOT write "default_model" or "mcpServers".
-	assert.NotContains(t, parsed, "default_model")
+	// Must write "default_model" (prevents Bedrock fallback) but NOT "mcpServers".
+	assert.Contains(t, parsed, "default_model")
 	assert.NotContains(t, parsed, "mcpServers")
 }