twitter: `post` says "tweet/thread" but only posts a single tweet — add thread support

## The gap

`twitter post` advertises itself as posting a **thread**:

```
post [ui] — Post a new tweet/thread
```

…but the implementation only fills `tweetTextarea_0` and submits once, so it always posts a **single** tweet. There's no first-class way to post a multi-tweet thread. (The name `twitter thread` is already taken by the *read* command that fetches an existing conversation.)

## Real-world pain (how we hit this)

We needed to post an 8-tweet launch thread. With the current CLI the only way was to script it by hand:

```bash
url=$(opencli twitter post "$(cat t1.txt)" --window foreground -f json | jq -r '.[0].url')
for i in 2 3 4 5 6 7 8; do
url=$(opencli twitter reply "$url" "$(cat t$i.txt)" --window foreground -f json | jq -r '.[0].url')
done
```

This works (it's exactly how an X thread is structured — each tweet replies to the previous), but every caller has to re-implement URL parsing, chaining, pacing, and failure handling. Worth making a first-class command.

## Proposed solution

A `twitter post-thread` command that takes N tweets (via `--file`, split on `---` lines or blank lines) and chains them: first tweet, then each as a reply to the previous, returning every id/url.

**Design question for maintainers** — which integration do you prefer?

1. **New `post-thread` command that reuses `twitter post` + `twitter reply` via the registry** (DRY, zero changes to the fragile Draft.js composer code). ← what our draft PR does
2. **Extend `post` itself** to accept multiple segments (fulfills its existing "tweet/thread" description), possibly refactoring the composer insert/submit into shared helpers first.
3. **Native modal thread** (`/compose/post` "Add post" + "Post all") for a single atomic submit, instead of reply-chaining.

We've opened a draft PR implementing option 1 (new command, reuses tested `post`/`reply`, unit-tested orchestration incl. abort-on-failure). Happy to reshape it toward 2 or 3 if you'd rather. 🙏

Start by reading the existing `twitter post` and `twitter reply` command implementations and the draft PR's orchestration tests. Done means a `post-thread` command accepts multiple tweet segments, chains replies, returns every id and URL, and aborts on failure as described.

Written by the indexing model from the issue text.