Docs / UX: what's the difference between "queued" and "pending" messages?

## Summary
The CLI surfaces two distinct concepts — "queued" messages and "pending" messages — without explaining how they differ. From the user's perspective they look like the same thing (a message you've typed but the agent hasn't processed yet).

## Questions to resolve
- Is there a meaningful behavioral difference (lifecycle, when each is flushed, when each can be cancelled)?
- If yes: please document it in `/help` and in the on-screen labels (e.g. tooltip on the queue indicator).
- If no: please unify the terminology to a single word in the UI.

## Why this matters
Existing issues about queued-message editing/cancellation (#2055, #2378, #2905, #1857, #3692) all use "queued"; some other surfaces use "pending." Users can't tell whether they're filing duplicate reports about the same thing or two genuinely different bugs.

まず CLI での "queued" と "pending" の使用箇所を比較し、次に /help のテキストと、issue で言及されている画面上のキューインジケーターのラベルを調べます。これらの用語が異なるライフサイクル、フラッシュ動作、またはキャンセル状態を表しているかどうかを判断し、その違いを /help と tooltip に記載するか、同義であれば用語を統一します。

索引モデルが issue の本文から書いたものです。