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

test/testdata/claude_code_sentinels/control_request_can_use_tool.json

@@ -0,0 +1,11 @@
+{
+  "type": "control_request",
+  "request_id": "req_123",
+  "request": {
+    "subtype": "can_use_tool",
+    "tool_name": "Bash",
+    "input": {"command": "npm test"},
+    "tool_use_id": "toolu_123",
+    "description": "Running npm test"
+  }
+}

test/testdata/claude_code_sentinels/session_state_changed.json

@@ -0,0 +1,7 @@
+{
+  "type": "system",
+  "subtype": "session_state_changed",
+  "state": "requires_action",
+  "uuid": "22222222-2222-4222-8222-222222222222",
+  "session_id": "sess_123"
+}

test/testdata/claude_code_sentinels/tool_progress.json

@@ -0,0 +1,10 @@
+{
+  "type": "tool_progress",
+  "tool_use_id": "toolu_123",
+  "tool_name": "Bash",
+  "parent_tool_use_id": null,
+  "elapsed_time_seconds": 2.5,
+  "task_id": "task_123",
+  "uuid": "11111111-1111-4111-8111-111111111111",
+  "session_id": "sess_123"
+}

test/testdata/claude_code_sentinels/tool_use_summary.json

@@ -0,0 +1,7 @@
+{
+  "type": "tool_use_summary",
+  "summary": "Searched in auth/",
+  "preceding_tool_use_ids": ["toolu_1", "toolu_2"],
+  "uuid": "33333333-3333-4333-8333-333333333333",
+  "session_id": "sess_123"
+}