Support merging threads (`hey merge`)

HEY's Merge Threads feature isn't currently exposed in the CLI.

## Requested

A command to merge two or more threads, e.g.:

```
hey merge [--title "New thread title"]
```

## Notes

- Since merges are irreversible, a `--confirm` flag or interactive prompt would be a nice safety net
- Similar to [#74](https://github.com/basecamp/hey-cli/issues/74) — may also require API support from HEY

Start by reviewing existing hey-cli command patterns and issue #74, then confirm whether HEY exposes an API operation for merging threads. Done means supporting multiple posting IDs, the optional title, and a confirmation safeguard for the irreversible action.

Written by the indexing model from the issue text.