feat(connectors): add /new to start a fresh chat from the app

A connection kept one chat per person for good, with no way to ask for
a new one. A person can now send /new (or !new, for Slack, which keeps
messages that begin with a slash for itself) as the whole message.

The host reads the command after the pairing check and before any
model turn, so the agent never sees it and the logs record only the
ids. It drops the link from the person, or from the shared thread or
group, to its chat. The old chat stays in Studio. The link is in the
shared store, so it works with several frontends and for every
connector add-on.

In the chat mode and in a thread of a group, everyone shares the chat,
so only a paired sender may reset it. The thread mode answers that
every message already starts a new chat. A chat with a running turn is
left alone and the sender is asked to repeat the command later.

Telegram did not treat /new@BotName as addressed to the bot in a
group; it does now, and keeps the text. The connection page gets one
help line under Conversations.

Assisted-by: Claude:claude-sonnet-5-5

docs/content/features/chats-connectors.md

@@ -397,6 +397,36 @@ that part.
 | `window` | As `sender`, but a message that comes `window_minutes` (1 to 10,080) or more after the same person's previous message starts a new chat. A thread keeps its chat. |
 | `thread` | Each message starts a new chat. A reply to an answer of the bot, or to the message that started a chat, goes on in that chat; on Slack, so does a thread under either one. |
 
+#### Start a new chat with `/new`
+
+A person can ask for a new chat at any time. The person sends `/new` as a message to the
+bot. Slack takes a message that begins with a slash for its own commands, so there the
+person sends `!new`. The host reads the command before any model turn: the agent never
+sees it, and the logs record only that a new chat started, with the ids, never the text.
+
+The command is the whole message. The case does not matter, and white space around it is
+ignored. Telegram's `/new@BotName` works when it names the bot. A message such as
+`/new please summarise` is plain text and goes to the agent. A message with a file is
+never a command. Only a person who may talk to the bot can use it; any other person gets
+nothing, as for any message.
+
+| Mode | What `/new` does |
+|---|---|
+| `sender`, `window` | The next message of the person starts a new chat. The old chat stays in Studio, with all its messages. |
+| `chat` | The next message of anyone in the group starts a new chat, for everyone. Only a sender that the owner paired with the bot may do this. Anyone else gets a short refusal. |
+| `thread` | Nothing to reset: the bot says that every message already starts a new chat. |
+
+In a thread of the platform (a Slack thread, a Telegram forum topic) the command resets
+only the chat of that thread. The people in a thread share its chat, so in a group a
+paired sender must send it. In a private chat the person can always send it.
+
+If the bot is still answering in that chat, it asks the person to send the command again
+when it is done, so a running turn never loses its chat. The old chat is not deleted, and
+memory and files stay as they are. A trigger whose delivery target is the origin chat
+still points at the old chat. The command needs no support from the add-on: it works for
+every connector, and with several frontends, because the link from a person to a chat is
+in the shared store.
+
 Some add-ons add settings of their own to a binding (for example Slack's **Threads the
 bot follows**). They appear under **How it answers**, and in the API as `options`, a
 map from the option id to a value. The add-on declares the allowed values, and an