feat: add direct port-forward diagnostic command

[acmore]

Summary

• add a new foreground okdev port-forward command for direct Kubernetes pod port-forwarding
• reuse session targeting and add focused validation/tests for single-pod local forwarding
• document the diagnostic command in the command reference

Test Plan

• go test ./internal/cli -count=1
• .venv/bin/pre-commit run --all-files --hook-stage manual okdev-gofmt