No agent-side hard stop: tts.clear for all modes; audio.clear does not stop queued tts.speak in audio mode #19

There is no reliable way for the agent to stop the bot talking immediately, in any mode.

Audio mode: audio.clear is acknowledged, but the sentence currently playing and any queued tts.speak sentences keep playing; only meeting.mic off silences them. audio.clear only clears the meeting bot's buffer, while the backend keeps sending the rest of the sentence and the queue.

Webpage modes: there is no agent command that clears playback and the backend TTS queue. The avatar page's own interruption only cuts the current utterance; if the reply was sent as several tts.speak commands, the next queued sentence fires tts.started, which reopens the page's audio gate, so the bot keeps talking.

Fix: a tts.clear command that works in every mode — drain the backend TTS queue, cancel in-flight playback, send audio.clear to the meeting bot (audio mode) and broadcast tts.audio_clear to the page (webpage modes). audio.clear in audio mode should behave the same way.

Reported in #15 and #16.