feat(down): add wait-for-termination flag

[acmore]

Summary

• add okdev down --wait and --wait-timeout so teardown can block until the workload object is gone and session pods have terminated
• report wait planning and results in down JSON output, and keep local cleanup running even if the termination wait times out
• document the new wait behavior in the command reference and quickstart

Testing

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