feat(chats): archive chats instead of hard-deleting them (#22406)

## 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)

coderd/database/models.go

@@ -3900,6 +3900,7 @@ type Chat struct {
 	ParentChatID      uuid.NullUUID `db:"parent_chat_id" json:"parent_chat_id"`
 	RootChatID        uuid.NullUUID `db:"root_chat_id" json:"root_chat_id"`
 	LastModelConfigID uuid.UUID     `db:"last_model_config_id" json:"last_model_config_id"`
+	Archived          bool          `db:"archived" json:"archived"`
 }
 
 type ChatDiffStatus struct {