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/fileset.go

@@ -6,8 +6,12 @@ type File struct {
 	Dest          string
 	SymlinkTarget string
 	Content       []byte
-	Mode          os.FileMode
 	Kind          Kind
+	Mode          os.FileMode
+	// NoMerge skips the 3-way git merge and always writes the rendered content.
+	// Use for files the adapter partially manages via JSON/YAML merge (e.g. live
+	// state files where the adapter already reconciles at the key level).
+	NoMerge bool
 }
 
 type FileSet struct {