refactor: remove dead code, deduplicate defaultHome, simplify mcpEntry + kilo render

Dead code removed:
- BuildProviders, buildModels, ProviderEntry (no production callers)
- ZedLanguageModels (shadowed by adapter-local version, wrong schema)
- GatewayProviderID deprecated constant (zero references)
- Corresponding dead tests for BuildProviders

Deduplication:
- Extract common.DefaultHome() to internal/adapter/common/home.go
- All 7 adapters call common.DefaultHome() instead of local copies (-49 lines)

Simplification:
- mcp.go: merge MCPClaudeStyle/MCPCrushStyle branches (identical URL path,
  one-line difference in stdio path) — clearer intent, fewer lines
- kilo.go: extract opencode.jsonc merge to mergeOpenCodeJSONC() helper,
  Render() drops from 92 to ~55 lines
- goose.go: strings.IndexByte → strings.Cut in splitProviderModel

internal/adapter/common/common_test.go

@@ -9,32 +9,6 @@ import (
 	"github.com/lukaszraczylo/harness-sync/internal/canonical"
 )
 
-func TestBuildProvidersGatewayFirst(t *testing.T) {
-	p := &canonical.Profile{
-		Gateway: canonical.Gateway{URL: "https://gw", Token: "t", DefaultModel: "x"},
-		Models:  []canonical.Model{{ID: "m1", Alias: "a1"}},
-		Upstreams: []canonical.Upstream{
-			{Name: "anthropic", APIKey: "k", BaseURL: "https://api.anthropic.com"},
-		},
-	}
-	out := BuildProviders(p)
-	require.Len(t, out, 2)
-	assert.Equal(t, "hs-gw", out[0]["id"])
-	assert.Equal(t, "anthropic", out[1]["id"])
-	assert.NotNil(t, out[0]["models"])
-}
-
-func TestBuildProvidersNoGateway(t *testing.T) {
-	p := &canonical.Profile{
-		Upstreams: []canonical.Upstream{
-			{Name: "anthropic", APIKey: "k"},
-		},
-	}
-	out := BuildProviders(p)
-	require.Len(t, out, 1)
-	assert.Equal(t, "anthropic", out[0]["id"])
-}
-
 func TestBuildMCPMap(t *testing.T) {
 	reg := &canonical.MCPRegistry{Servers: []canonical.MCPServer{
 		{Name: "filepuff", Command: "/bin/x", Transport: "stdio"},