fix: preserve worker task failure details

[thenderson21]

Summary

• snapshot terminal worker failure details before automatic Task reclamation
• provide deterministic diagnostics for invalid worker termination and return shape
• keep filesystem worker capability denied while its host binding remains shared

Validation

• ./build.sh
• ./test-aivm-c.sh
• aivm_test_worker_runtime
• aivm_test_vm_worker_tasks
• default Weather worker failure now reports the denied sys.fs.path.exists capability

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