[Fix] Web Fast sessions post duplicate lifecycle replies

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Web Fast Sessions now use completed native assistant output as their canonical reply instead of exposing send_chat_reply or requiring chat-style acknowledgements.
• Native web replies retain OpenCode session and message identity, participate in retry replacement and compatibility persistence, and continue to render through the existing Session transcript path.
• Web launch_task calls use the visible task event without posting a second kickoff message. Chat destinations still require and deliver their kickoff text.

Why this change was made

Web Sessions already persist the model's completed assistant output and have no external reply destination, so routing their communication through a chat delivery tool added an unnecessary lifecycle contract and duplicate launch narration. This is the architectural follow-up and intentionally remains separate from transcript-only workaround behavior.

Impact

Web Fast conversations produce one native final response and one visible task launch event. Slack, Discord, Microsoft Teams, Telegram, automation reports, structured user input, coding tasks, and child-task-to-parent relays retain their existing behavior.

[roomote-community]

No code issues found. See task

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