cli: plumb --log-level into event filter (or drop the flag)

[bilby91]

The devcontainer CLI exposes --log-level info|debug|trace as a global flag, but nothing reads it today — every event from the engine renders at the same verbosity in cmd/devcontainer/events.go.

Two ways forward:

1. Filter events in printEvent based on level (e.g. BuildLogEvent / LifecycleOutputEvent only at debug+). Cheap, CLI-local, no library change.
2. Wait until the library grows a verbosity knob on EngineOptions and honor that instead.

Lowest-friction option is (1). Until either lands, the flag is misleading and should either be removed or documented as a no-op.