docs(telegram): document /topic multi-session DM mode

Adds a new section 'Multi-session DM mode (/topic)' to the Telegram
messaging docs, covering:

- Comparison table vs the existing config-driven extra.dm_topics
- BotFather prerequisites (Threads Settings, user-create permission)
- Activation flow and root-DM lobby behavior
- End-user flow for creating topics via the + button / All Messages
- Auto-renaming when Hermes generates session titles
- /new semantics inside a topic
- /topic <session-id> restore of previous sessions
- Persistence layout (SQLite side tables)
- How to disable the feature

Also:
- New /topic row in the messaging slash-commands reference
- Updated Bot API 9.4 summary to point at both topic features

website/docs/reference/slash-commands.md

@@ -145,6 +145,7 @@ The messaging gateway supports the following built-in commands inside Telegram,
 | `/undo` | Remove the last exchange. |
 | `/sethome` (alias: `/set-home`) | Mark the current chat as the platform home channel for deliveries. |
 | `/compress [focus topic]` | Manually compress conversation context. Optional focus topic narrows what the summary preserves. |
+| `/topic [session-id]` | **Telegram DM only.** Enable or inspect user-managed multi-session topic mode. See [Multi-session DM mode](/docs/user-guide/messaging/telegram#multi-session-dm-mode-topic). |
 | `/title [name]` | Set or show the session title. |
 | `/resume [name]` | Resume a previously named session. |
 | `/usage` | Show token usage, estimated cost breakdown (input/output), context window state, session duration, and — when available from the active provider — an **Account limits** section with remaining quota / credits pulled live from the provider's API. |
@@ -174,6 +175,6 @@ The messaging gateway supports the following built-in commands inside Telegram,
 
 - `/skin`, `/snapshot`, `/gquota`, `/reload`, `/tools`, `/toolsets`, `/browser`, `/config`, `/cron`, `/skills`, `/platforms`, `/paste`, `/image`, `/statusbar`, `/plugins`, `/busy`, `/indicator`, `/redraw`, `/clear`, `/history`, `/save`, `/copy`, and `/quit` are **CLI-only** commands.
 - `/verbose` is **CLI-only by default**, but can be enabled for messaging platforms by setting `display.tool_progress_command: true` in `config.yaml`. When enabled, it cycles the `display.tool_progress` mode and saves to config.
-- `/sethome`, `/update`, `/restart`, `/approve`, `/deny`, and `/commands` are **messaging-only** commands.
+- `/sethome`, `/update`, `/restart`, `/approve`, `/deny`, `/topic`, and `/commands` are **messaging-only** commands.
 - `/status`, `/background`, `/queue`, `/steer`, `/voice`, `/reload-mcp`, `/rollback`, `/debug`, `/fast`, `/footer`, `/curator`, `/kanban`, and `/yolo` work in **both** the CLI and the messaging gateway.
 - `/voice join`, `/voice channel`, and `/voice leave` are only meaningful on Discord.