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_test.go

@@ -0,0 +1,46 @@
+package executor
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+func TestClaudeBuiltinToolRegistry_DefaultSeedFallback(t *testing.T) {
+	registry := augmentClaudeBuiltinToolRegistry(nil, nil)
+	for _, name := range defaultClaudeBuiltinToolNames {
+		if !registry[name] {
+			t.Fatalf("default builtin %q missing from fallback registry", name)
+		}
+	}
+}
+
+func TestApplyClaudeToolPrefix_KnownFallbackBuiltinsRemainUnprefixed(t *testing.T) {
+	for _, builtin := range defaultClaudeBuiltinToolNames {
+		t.Run(builtin, func(t *testing.T) {
+			input := []byte(fmt.Sprintf(`{
+				"tools":[{"name":"Read"}],
+				"tool_choice":{"type":"tool","name":%q},
+				"messages":[{"role":"assistant","content":[{"type":"tool_use","name":%q,"id":"toolu_1","input":{}},{"type":"tool_reference","tool_name":%q},{"type":"tool_result","tool_use_id":"toolu_1","content":[{"type":"tool_reference","tool_name":%q}]}]}]
+			}`, builtin, builtin, builtin, builtin))
+			out := applyClaudeToolPrefix(input, "proxy_")
+
+			if got := gjson.GetBytes(out, "tool_choice.name").String(); got != builtin {
+				t.Fatalf("tool_choice.name = %q, want %q", got, builtin)
+			}
+			if got := gjson.GetBytes(out, "messages.0.content.0.name").String(); got != builtin {
+				t.Fatalf("messages.0.content.0.name = %q, want %q", got, builtin)
+			}
+			if got := gjson.GetBytes(out, "messages.0.content.1.tool_name").String(); got != builtin {
+				t.Fatalf("messages.0.content.1.tool_name = %q, want %q", got, builtin)
+			}
+			if got := gjson.GetBytes(out, "messages.0.content.2.content.0.tool_name").String(); got != builtin {
+				t.Fatalf("messages.0.content.2.content.0.tool_name = %q, want %q", got, builtin)
+			}
+			if got := gjson.GetBytes(out, "tools.0.name").String(); got != "proxy_Read" {
+				t.Fatalf("tools.0.name = %q, want %q", got, "proxy_Read")
+			}
+		})
+	}
+}