v1.18.40: Unified sandbox exec output limits

Highlights

• Report outputLimited consistently from sandbox exec backends.
• Terminate native sandbox commands when captured output exceeds the backend cap.
• Bound Tokimo and AgentRun exec output before surfacing tool results.
• Document the v1.18.40 source line and add regression coverage.

Validation

• git diff --check
• php scripts/sdk-bc-check.php --verify
• ./vendor/bin/phpunit tests/Sdk/SandboxTest.php tests/Sdk/TokimoSandboxBackendTest.php tests/Sdk/AgentRunSandboxBackendTest.php tests/Unit/BashToolTest.php
• composer validate --strict
• composer audit --format=plain
• composer run lint
• composer test
• GitHub Actions main and tag test runs passed.