fix(adapters): ground native schemas, merge-not-replace, add goose/cagent/zed

- common/mcp.go: MCPStyle enum + BuildMCPMapStyled for claude/crush/opencode/zed dialects
- common/jsonmerge.go: MergeJSONKeys (JSON-first, JSONC fallback) preserves user keys;
  fix: try plain json.Unmarshal before StripJSONComments to avoid mangling URLs with //
- common/yamlmerge.go: MergeYAMLKeys for YAML configs (goose)
- common/blocks.go: add ProvidersAsMap for opencode map-shaped provider key
- claudecode: merge settings.json (preserves hooks/permissions/env); MCPClaudeStyle (no type field)
- crush: fix mcp key (was mcpServers); MCPCrushStyle entries include type field; merge crush.json; fix import key mcp
- kilo: drop providers key (kilo has none); MCPOpencodeStyle (local/remote); merge kilo.json
- opencode: fix provider (singular map) + model (not default_model); MCPOpencodeStyle; merge jsonc; fix import key mcp
- goose: new adapter — YAML merge, extensions map, splitProviderModel, import reconstructs MCP from extensions
- cagent: new adapter — renders starter default.yaml, import returns empty
- zed: new adapter — merge settings.json, context_servers (MCPZedStyle) + agent.default_model
- docs: append Grounded harness schemas section to design spec

internal/adapters/opencode/import.go

@@ -16,7 +16,7 @@ func importFrom(home string) (*adapter.ImportResult, error) {
 		return nil, err
 	}
 	if cfgBody != "" {
-		mcpServers, mcpErr := common.ParseMCPFromJSON([]byte(cfgBody), "mcpServers")
+		mcpServers, mcpErr := common.ParseMCPFromJSON([]byte(cfgBody), "mcp")
 		if mcpErr != nil {
 			return nil, mcpErr
 		}

internal/adapters/opencode/opencode.go

@@ -8,7 +8,6 @@ import (
 	"github.com/lukaszraczylo/harness-sync/internal/adapter"
 	"github.com/lukaszraczylo/harness-sync/internal/adapter/common"
 	"github.com/lukaszraczylo/harness-sync/internal/canonical"
-	"github.com/lukaszraczylo/harness-sync/internal/render"
 )
 
 const name = "opencode"
@@ -49,18 +48,30 @@ func (a *Adapter) Detect() bool {
 }
 
 // Render produces the FileSet that applies the canonical bundle to opencode.
+// opencode.jsonc is MERGED to preserve user-managed keys ($schema, agent,
+// instructions, …). Correct keys: "provider" (singular, map), "model" (string),
+// "mcp" (map with type: local|remote entries).
 func (a *Adapter) Render(b *canonical.Bundle) (*adapter.FileSet, error) {
 	fs := adapter.NewFileSet()
 	base := filepath.Join(a.home, ".config", "opencode")
 
-	cfg, err := renderConfig(b)
+	cfgPath := filepath.Join(base, "opencode.jsonc")
+	existing, _ := os.ReadFile(cfgPath)
+	overlay := map[string]any{
+		"provider": common.ProvidersAsMap(&b.Profile),
+		"model":    b.Profile.Gateway.DefaultModel,
+	}
+	if mcp := common.BuildMCPMapStyled(&b.MCP, common.MCPOpencodeStyle); len(mcp) > 0 {
+		overlay["mcp"] = mcp
+	}
+	merged, err := common.MergeJSONKeys(existing, overlay)
 	if err != nil {
 		return nil, err
 	}
 	fs.Add(adapter.File{
-		Dest:    filepath.Join(base, "opencode.jsonc"),
+		Dest:    cfgPath,
 		Kind:    adapter.RenderedFile,
-		Content: cfg,
+		Content: merged,
 	})
 
 	instructions := b.Instructions.Global
@@ -76,17 +87,6 @@ func (a *Adapter) Render(b *canonical.Bundle) (*adapter.FileSet, error) {
 	return fs, nil
 }
 
-func renderConfig(b *canonical.Bundle) ([]byte, error) {
-	out := map[string]any{
-		"providers":     common.BuildProviders(&b.Profile),
-		"default_model": b.Profile.Gateway.DefaultModel,
-	}
-	if mcp := common.BuildMCPMap(&b.MCP); len(mcp) > 0 {
-		out["mcpServers"] = mcp
-	}
-	return render.JSON(out)
-}
-
 // Import reads opencode config from home and returns a canonical ImportResult.
 func (a *Adapter) Import(home string) (*adapter.ImportResult, error) {
 	return importFrom(home)

internal/adapters/opencode/opencode_test.go

@@ -34,12 +34,81 @@ func TestOpencodeRender(t *testing.T) {
 	assert.Equal(t, adapter.RenderedFile, seen[cfgDest].Kind)
 	var parsed map[string]any
 	require.NoError(t, json.Unmarshal(seen[cfgDest].Content, &parsed))
+	// Correct key is "provider" (singular map), not "providers" (slice).
+	assert.Contains(t, parsed, "provider")
+	assert.NotContains(t, parsed, "providers")
+	// Correct key is "model", not "default_model".
+	assert.Contains(t, parsed, "model")
+	assert.NotContains(t, parsed, "default_model")
+	// No "mcpServers" — opencode uses "mcp".
+	assert.NotContains(t, parsed, "mcpServers")
 
 	agentsDest := filepath.Join(home, ".config", "opencode", "AGENTS.md")
 	assert.Equal(t, adapter.RenderedFile, seen[agentsDest].Kind)
 	assert.Contains(t, string(seen[agentsDest].Content), "# global")
 }
 
+func TestOpencodeRenderWithMCP(t *testing.T) {
+	home := t.TempDir()
+	ad := New(WithHome(home))
+	b := &canonical.Bundle{
+		Profile: canonical.Profile{
+			Gateway: canonical.Gateway{DefaultModel: "sonnet"},
+		},
+		MCP: canonical.MCPRegistry{Servers: []canonical.MCPServer{
+			{Name: "fp", Command: "/bin/fp"},
+		}},
+	}
+	fs, err := ad.Render(b)
+	require.NoError(t, err)
+	seen := map[string]adapter.File{}
+	fs.ForEach(func(f adapter.File) { seen[f.Dest] = f })
+
+	cfgDest := filepath.Join(home, ".config", "opencode", "opencode.jsonc")
+	var parsed map[string]any
+	require.NoError(t, json.Unmarshal(seen[cfgDest].Content, &parsed))
+	mcp := parsed["mcp"].(map[string]any)
+	fp := mcp["fp"].(map[string]any)
+	// MCPOpencodeStyle: type local, command as array, enabled true.
+	assert.Equal(t, "local", fp["type"])
+	assert.Equal(t, true, fp["enabled"])
+	cmd, _ := fp["command"].([]any)
+	assert.Equal(t, "/bin/fp", cmd[0])
+}
+
+func TestOpencodeRenderMergesExistingKeys(t *testing.T) {
+	home := t.TempDir()
+	base := filepath.Join(home, ".config", "opencode")
+	require.NoError(t, os.MkdirAll(base, 0o750))
+
+	existing := map[string]any{
+		"$schema": "https://opencode.schema/v1",
+		"agent":   map[string]any{"maxTokens": 8192},
+	}
+	body, _ := json.Marshal(existing)
+	require.NoError(t, os.WriteFile(filepath.Join(base, "opencode.jsonc"), body, 0o600))
+
+	ad := New(WithHome(home))
+	b := &canonical.Bundle{
+		Profile: canonical.Profile{
+			Gateway: canonical.Gateway{DefaultModel: "sonnet"},
+		},
+	}
+	fs, err := ad.Render(b)
+	require.NoError(t, err)
+	seen := map[string]adapter.File{}
+	fs.ForEach(func(f adapter.File) { seen[f.Dest] = f })
+
+	cfgDest := filepath.Join(base, "opencode.jsonc")
+	var parsed map[string]any
+	require.NoError(t, json.Unmarshal(seen[cfgDest].Content, &parsed))
+	// User keys preserved.
+	assert.Equal(t, "https://opencode.schema/v1", parsed["$schema"])
+	assert.Contains(t, parsed, "agent")
+	// Managed keys overlaid.
+	assert.Contains(t, parsed, "model")
+	assert.Contains(t, parsed, "provider")
+}
 func TestOpencodeImportStripsComments(t *testing.T) {
 	home := t.TempDir()
 	base := filepath.Join(home, ".config", "opencode")