Avoid runtime pipe backpressure

[GHX5T-SOL]

/claim #1739

Summary

• redirect agent runtime stdout and stderr to DEVNULL instead of leaving unmanaged pipes attached
• add a regression proving a child can write large stdout/stderr output without deadlocking and that no pipe handles are exposed on the Popen object
• preserve STOPPED state after intentional runtime stops and include small current-main suite-health fixes for AgentStatus export and metrics timer locking

Validation

• python3 -m pytest tests/test_runtime.py tests/test_metrics.py tests/test_agent_registry.py -q -> 14 passed
• timeout 60s python3 -m pytest -q -> 24 passed
• python3 -m flake8 src/agent/runtime.py src/agent/init.py src/common/metrics.py tests/test_runtime.py -> passed
• python3 -m py_compile src/agent/runtime.py src/agent/init.py src/common/metrics.py tests/test_runtime.py tests/test_metrics.py -> passed
• git diff --check -> passed
• git diff | gitleaks detect --pipe --redact --no-banner -> no leaks
• star gate verified: viewerHasStarred=true

Fixes #1739