Add worker cancellation controls

[ianhodge]

Summary

• handle task_cancellation WebSocket control messages and cancel matching active task contexts
• report cancelled tasks with terminal task_state=CANCELLED while preserving the existing completion flow
• record cancellation metrics and lifecycle events for server-initiated and shutdown cancellations
• harden Docker/direct cancellation behavior and update README cancellation metric docs

Validation

• go test ./internal/metrics ./internal/worker
• go test ./...
• helm template oz-agent-worker charts/oz-agent-worker --set image.tag=test --set worker.workerId=test-worker --set warp.apiKeySecret.create=true --set warp.apiKeySecret.value=dummy --set metrics.enabled=true
• git diff --check

Co-Authored-By: Oz oz-agent@warp.dev

[ianhodge]

• Add worker cancellation controls #63 👈 (View in Graphite)
• Add worker observability metrics #62
• main

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