#3306 feat(panel,host): the windows' inbox reads become pushed follows, and the held inbox reads are retired

The window's inbox reads become pushed follows, and the held inbox reads are retired. This is stacked on #3264.

What changes

• Panel: the Inbox tab, the Inbox page, Home and Factory hold one codingAgents.inboxFollow stream per plan or agent they show (use-followed-inboxes.ts). Each holds the items as each change arrives.
  • There's no read loop or cursor any more.
  • "Mark all read" no longer re-reads, because each change carries the reader's own read markers.
• Currency: a plan counts as current once its pushed items are as new as main's word of it. So a link to a brand-new item waits for it rather than saying "Not found".
• Retired: the held reads codingAgents.inbox and codingAgents.agentInbox are removed, together with their act-port kinds and procedures, Gateway methods, host calls, and parameter schemas.

Tests

• testing/followed-inboxes.ts stands in for the host's inbox streams.
• Rewritten on it: use-inbox-items.test.ts, the inbox tests in desktop-inbox, desktop-home, desktop-factory, person-page, inbox-link-chips, inbox-side-panel, read-failure, desktop-tasks and composer-drafts, and use-inbox-read-marks.
• Tests of behaviour that no longer exists were dropped, such as an older read being overtaken or held reads diverging.

To check by hand

1. Open a plan's Inbox tab. Its items show. Answer one, and the card updates.
2. Have an agent post an item. Within about a second, it shows in the Inbox tab, the Inbox page, Home and Factory, without reopening anything.
3. Click a notification for a new item. The Inbox opens on it, with no "Not found".