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.go

@@ -23,9 +23,22 @@ func (k Kind) String() string {
 	return "unknown"
 }
 
+// HarnessCapabilities declares what an adapter manages.
+// HasBuiltInSub means the harness has its own AI subscription (e.g. Claude Max)
+// and harness-sync should not write provider/model/endpoint config for it.
+type HarnessCapabilities struct {
+	ManagesProviders    bool // writes provider/gateway endpoint config
+	ManagesModels       bool // writes model selection config
+	ManagesMCP          bool // writes MCP server config
+	ManagesSkills       bool // writes skills symlink or equivalent
+	ManagesInstructions bool // writes instructions file (CLAUDE.md, AGENTS.md, etc.)
+	HasBuiltInSub       bool // has its own subscription; provider/model config is skipped
+}
+
 type Adapter interface {
 	Name() string
 	Detect() bool
+	Capabilities() HarnessCapabilities
 	Render(b *canonical.Bundle) (*FileSet, error)
 	Import(home string) (*ImportResult, error)
 }