Summary

The UI has always labeled the action as "Archive agent" but the backend was performing a hard DELETE, permanently destroying chats and all their messages.

This change replaces the hard delete with a soft archive, consistent with the pattern used by template versions.

Changes

Database

• Migration 000423: Add archived boolean DEFAULT false NOT NULL column to chats table
• Replace DeleteChatByID query with ArchiveChatByID (UPDATE SET archived = true)
• Add UnarchiveChatByID query (UPDATE SET archived = false)
• Filter archived chats from GetChatsByOwnerID (WHERE archived = false)

API

• Remove DELETE /api/experimental/chats/{chat}
• Add POST /api/experimental/chats/{chat}/archive — archives a chat and all its descendants
• Add POST /api/experimental/chats/{chat}/unarchive — unarchives a single chat (API only, no UI yet)

Backend

• archiveChatTree() recursively archives child chats (replaces deleteChatTree() which hard-deleted)
• Chat daemon's ArchiveChat() archives the full chat tree in a transaction
• Authorization uses ActionUpdate instead of ActionDelete

SDK

• Replace DeleteChat() with ArchiveChat() and UnarchiveChat()
• Add Archived field to Chat struct

Frontend

• archiveChat API call uses POST .../archive instead of DELETE
• No UI changes — the "Archive agent" button now actually archives instead of deleting

Design Decision

This follows the template version archive pattern (Pattern B in the codebase):

• archived boolean column (not deleted boolean)
• Dedicated POST .../archive and POST .../unarchive routes (not repurposing DELETE)
• Reversible — users can unarchive via the API (UI for this will come later)