simulate: audio subcommand with impairment flags

[u9g]

Turns --audio into an audio subcommand, which carries three new booleans sent on the create-simulation message:

lk agent simulate audio \
  --scenarios scenarios.yaml \
  --background-noise \
  --low-quality-microphone \
  --packet-loss

simulate's own flags did not need duplicating on the subcommand: urfave/cli v3 flags are persistent unless Local is set, so --scenarios, --agent-name, -n and friends parse on either side of audio. Since v3's help only renders the root's persistent flags under GLOBAL OPTIONS, the subcommand carries a one-line description pointing back at them.

Verified against a local fake twirp endpoint: lk agent simulate audio --background-noise --low-quality-microphone --packet-loss sends

mode: SIMULATION_MODE_AUDIO
background_noise: true
low_quality_microphone: true
packet_loss: true

while plain lk agent simulate still sends SIMULATION_MODE_TEXT with no impairments, and the impairment flags are rejected on the parent command.

The protocol side (livekit/protocol#1723) has merged, so the temporary local-checkout build is gone and go.mod points at the real fields.

Two dependency notes on that bump, both pseudo-versions because neither repo has cut a release containing the commits yet:

• github.com/livekit/protocol → v1.50.5-0.20260824151414-2122e1d08bc9 (protocol main at the #1723 squash). The open release PR is Version Packages protocol#1683.
• github.com/livekit/server-sdk-go/v2 → v2.18.2-0.20260824093707-96a978c6d455. This one is not optional and unrelated to this PR: protocol#1730 changed TransferSIPParticipant to return a response instead of emptypb.Empty, and the published v2.18.1 cannot compile against protocol main. server-sdk-go#971 fixed it on their main. Both pins can collapse to tagged versions once protocol and server-sdk-go publish.

Breaking: --audio shipped in v2.18.2 and is removed by this change, replaced by the subcommand. Say the word if it should stay as a hidden alias.