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/mcp.go

@@ -12,7 +12,7 @@ const (
 	MCPCrushStyle
 	// MCPOpencodeStyle: kilo + opencode shape — type: local|remote, command as []string.
 	MCPOpencodeStyle
-	// MCPZedStyle: zed context_servers — {enabled, source, command, args} or {enabled, url}.
+	// MCPZedStyle: zed context_servers — {enabled, command, args} or {enabled, url}.
 	MCPZedStyle
 )
 
@@ -94,11 +94,12 @@ func mcpEntry(s canonical.MCPServer, style MCPStyle) map[string]any {
 		}
 		e["enabled"] = true
 	case MCPZedStyle:
-		e["enabled"] = true
+		// Do NOT emit "enabled" — Zed 1.3.x has a serde flatten+deny_unknown_fields
+		// bug where outer-struct fields bleed into the untagged enum variants and
+		// fail all of them. Omitting "enabled" defaults to true.
 		if s.URL != "" {
 			e["url"] = s.URL
 		} else {
-			e["source"] = "custom"
 			if s.Command != "" {
 				e["command"] = s.Command
 			}