Keep Claude Code compatibility work low-risk and reviewable

This change stops short of broader Claude Code runtime alignment and instead
hardens two safe edges: builtin tool prefix handling and source-informed
sentinel coverage for future drift checks.

Constraint: Must preserve existing default behavior for current users
Rejected: Implement control-plane/session alignment now | too much runtime risk for a first slice
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Treat the new fixtures as compatibility sentinels, not a full Claude Code schema contract
Tested: go test ./test/...; go test ./sdk/translator/...; go test ./internal/runtime/executor -run 'Claude|Builtin|Tool'; go test ./...
Not-tested: End-to-end Claude Code direct-connect/session runtime behavior

internal/runtime/executor/claude_builtin_tools.go

@@ -0,0 +1,38 @@
+package executor
+
+import "github.com/tidwall/gjson"
+
+var defaultClaudeBuiltinToolNames = []string{
+	"web_search",
+	"code_execution",
+	"text_editor",
+	"computer",
+}
+
+func newClaudeBuiltinToolRegistry() map[string]bool {
+	registry := make(map[string]bool, len(defaultClaudeBuiltinToolNames))
+	for _, name := range defaultClaudeBuiltinToolNames {
+		registry[name] = true
+	}
+	return registry
+}
+
+func augmentClaudeBuiltinToolRegistry(body []byte, registry map[string]bool) map[string]bool {
+	if registry == nil {
+		registry = newClaudeBuiltinToolRegistry()
+	}
+	tools := gjson.GetBytes(body, "tools")
+	if !tools.Exists() || !tools.IsArray() {
+		return registry
+	}
+	tools.ForEach(func(_, tool gjson.Result) bool {
+		if tool.Get("type").String() == "" {
+			return true
+		}
+		if name := tool.Get("name").String(); name != "" {
+			registry[name] = true
+		}
+		return true
+	})
+	return registry
+}