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/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"
+}