A variant group is one `choose` item in the feed that opens its deck #1858

The feed now shows a variant group as one choose item that links to its deck, instead of one post per sibling pull. bun x tsc --noEmit, the new test and the existing feed tests, test/check-list.test.ts and test/variants-deck.test.ts pass here. I haven't run the whole suite, and the app wasn't booted or tried in a browser.

• Asking: while two or more siblings are open and none has shipped, the item reads "choose among N open variants of “<first pull's title>”". It links to /<owner>/<repo>/variants/<group> and ranks above plain news.
• Decided: once a sibling ships, or all but one close, it reads "decided" or "no variants are open". It carries zero urgency and asks nothing. It stays in the feed as a quiet item that decays like other news.
• Who sees it: only people who can write the repository, because the deck's votes are theirs to cast. Sealed pulls are left out. A muted app still shows a choose item that is asking, as with asks and milestones; a decided one is muted like any other.
• Query count: one grouped SQL read over the viewer's writable repos, newest 50 groups, so it doesn't grow with the number of siblings. It lives in a new src/feed/choose.ts, called from src/feed/items.ts. The mute exception is in src/feed/feed.ts, and the feed card badge reads "Choose".
• Tests: test/feed-choose.test.ts is added to GATE in scripts/gate.ts. It covers:

- two siblings give one asking item, and a reader without write access sees none; - a shipped sibling leaves no asking item; - a group with every sibling closed reads decided; - the query count is the same with 2 siblings as with 14.

I left out a consequence score and the ask payload (the structured options and "if nothing" text the real asks carry). This item is a pointer to the deck, with a fixed urgency of 3: below a failed run (4) and below real asks (5 and up). A group left with exactly one open sibling and nothing shipped reads decided, because there is nothing left to choose between. I added no new page, so there are no reserved names to update.

Closes #1805

The feed now shows a variant group as one Choose item that links to its deck. I added no terminal feed, so that part of the review isn't done. bun x tsc --noEmit, test/check-list.test.ts and the feed tests pass. I haven't run the whole suite, and the app wasn't booted or tried in a browser.

• Asks and goes: while two or more siblings are open and none has shipped, it reads "choose among N open variants of “<first pull's title>”". It is one grouped SQL read, so the query count doesn't grow with the siblings. Once a sibling ships, or all but one close, the item leaves the feed. Decided groups are filtered out in the query (HAVING), so they no longer pile up as zero-urgency rows. This changes the original issue's "reads decided" wording to "gone".
• No Mute button: a choose item gets none, like asks and milestones, since muting the app never silenced an open choice anyway (src/web/studio/feed.ts, src/feed/feed.ts).
• Both doors: the terminal has no feed (the README already says so), so there's nothing to add there. I didn't build a terminal feed for this. The item is in GET /api/v1/feed and the MCP feed tool as kind choose. The README's "Choosing among variants" section now says so, and a test fetches it through the API.
• Tests: test/feed-choose.test.ts is in GATE. It covers:

- the exact wording of the item; - a shipped sibling removes it; - all siblings closed removes it; - one sibling left open removes it; - muting the app leaves an open choice in place; - someone without write access sees none; - the query count is the same with 2 siblings as with 14.

I removed void name, the unused parameter and the as any casts.

Not done: no consequence score or ask payload on the item. Its urgency is a fixed 3, below failed runs and real asks.