[AbirAbbas]

Reverts #2472. That change reopened #2471.

Why

#2472 let the idle sweep release an open desktop view once it is outside the 3 most recently used. A regression check found that this lets another process run a turn in a chat the window is still showing. The window then misses that turn's content.

Measured with a standalone codeaf app (app/v3 2c717c0) on a scratch home, driven over its wire:

• Setup: 5 chats created and opened. After the idle sweep, the 2 oldest were released and the 3 most recent stayed held, as designed.
• Turn from the CLI: codeaf chat --once --session <released chat> on the same home ran (exit 0). Before Let idle desktop chat views beyond the three most recent go #2472 the same command is refused with "this conversation is open in another window". The control run against a still-held chat on the same binary refused it in exactly that way.
• What the window got: for that outside turn, the app engine emitted only agent.changed working → idle. It sent no turn.* and no entry.added, and updated_at did not move. The CLI process holds no engine host, so the app cannot join it, and the window shows nothing of the turn until it re-reads.
• Sending afterwards: a send from the app into that chat worked (turn.done ok:true). On reopening the chat, though, the app then started an extra turn on its own: "The app closed unexpectedly while you were in the middle of answering…".
• Same-engine sources are not affected: a second window of the same app and in-engine factory runs reopen the chat in the app engine and stream normally.

Rather than ship that, this restores the held view until #2471 has a fix that bounds the per-chat files and keeps the refusal. Possible directions:

• a desk "view closed" call, so the engine knows when a chat is no longer shown;
• keeping the hold but shedding the presence watch's per-file kqueue handles on macOS, which are 6 of about 7 fds per chat.

Change

git revert of 2c717c0, plus a change entry. The code, test and manual return to their state before #2472.

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