Add /new logical conversation reset and remove /reset duplicate

[louis4li]

Problem

/clear and /reset currently have the same effective behavior: they clear retained conversation history. That duplicate surface is ambiguous because /reset reads like a full state reset, while the existing conversation contract pins the Agent Profile snapshot for the lifetime of a logical conversation. Clearing only the pinned Profile inside an existing conversation would mix old task state, late completions, approvals, tool results, and new default Profile selection in one authority boundary.

Acceptance Criteria

• /clear remains the command for clearing the current logical conversation context/history without changing the pinned Agent Profile.
• /reset is no longer a duplicate history-clear command.
• /new starts a new logical backend conversation for the same Telegram/channel chat window, preserving channel binding and NyxID login binding.
• A new logical conversation starts with empty retained context and no pinned Agent Profile, so the next profiled run uses the currently effective default binding.
• Late completions from runs started in the old logical conversation continue to belong to the old conversation and cannot repin or modify the new conversation.
• Focused tests cover /reset removal and /new conversation rotation behavior.

Non-goals

• Do not cancel in-flight old conversation tasks as part of /new.
• Do not implement summary carry-over between old and new conversations.
• Do not change Telegram account binding or NyxID authorization binding semantics.
• Do not make /reset clear the pinned Profile inside an existing conversation.