feat(slack): Add support for handling message_deleted, message_changed - #549

## summary

adds `onMessageUpdated` and `onMessageDeleted`, so a bot can react when
a message is edited or removed. Slack dispatches both today; other
adapters can opt in later

supersedes #549, which was verified there against real Slack webhooks.
reopened from a branch in this repo with the original commits preserved
and signed

```typescript
bot.onMessageUpdated(async (thread, message, previousMessage) => {
  await mirror.update(message.id, message.text);
});

bot.onMessageDeleted(async (event) => {
  await mirror.remove(event.messageId);
});
```

both are lifecycle events: they never route through `onNewMessage`,
`onNewMention`, or `onSubscribedMessage`, and the concurrency strategies
do not apply

### notes

- **the bot's own edits are filtered.** slack sends a `message_changed`
for every `chat.update`, and post-and-edit streaming calls it once per
delta, so without this a single streamed reply would call the handler
back repeatedly on its own message
- **`previousMessage` is forwarded on edits.** slack sends the pre-edit
message and it was being dropped. an edit handler usually needs the
before to know what changed, so it is the optional third argument
- **the two shapes differ deliberately.** an edit carries a full
replacement message, so it gets `(thread, message, previousMessage?)`. a
delete has no message, only the id of what was removed, so it gets an
event. use `chat.thread(event.threadId)` when a delete handler needs one
- **one thread id helper** now serves message, edit, and delete, so an
edit cannot resolve to a different thread than the message it edits

## test plan

core:

- an edit dispatches to `onMessageUpdated` and not to the normal message
handlers
- the handler receives the pre-edit message as its third argument
- the bot's own edits are skipped
- a delete dispatches with normalized event data
- both run inside the active conversation, so read tools built in these
handlers stay scoped

slack:

- `message_changed` dispatches as an update, `message_deleted` as a
delete
- `previous_message` is forwarded, and left undefined when slack omits
it
- hidden unfurl updates stay ignored, hidden real edits still dispatch
- message, edit, and delete resolve to one thread id in a flat DM and in
a threaded `agent_view` DM

verified against a real slack workspace over socket mode: editing and
deleting a DM both routed to the same thread id as the original message

---------

Co-authored-by: Miłosz Lenczewski <m.lenczewski@tidio.net>