[AbirAbbas]

Fixes #2471.

Problem

Every chat the desktop app shows stayed held by codeaf app until the app quit, along with its open files and per-chat timers.

• The window calls agents.open when it shows a chat, including a hover prefetch. It calls agents.close only for a factory job's chats.
• agents.open sets viewOpen, and the idle sweep never let a conversation with viewOpen go. viewOpen was cleared only by agents.close. Merge dev (through #2002) into app/v3: relay, two-computer fixes, quit saving #2009 added this so that an open view keeps its relay hold.
• So the holds grew with the number of chats shown, not with time. The macOS soak in app: each new chat keeps ~10 files open in the engine for the life of the app (macOS soak) #2471 measured engine fds of 59 → 74 → 85 → ~96 over three chats.

What one held chat costs, measured with lsof on a standalone codeaf app driven over its wire:

• 6 fds from the presence watch on the session folder: fsnotify's kqueue backend opens the folder and each entry in it (.cell, .codeaf, meta.json, presence.json, meta.lock).
• 1 fd for the journal, .cell/transcript.jsonl.
• Once for all held chats: the watcher's kqueue and its two pipes.
• The SQLite stores (4 handles, 7 WAL/shm) are process-wide and do not grow per chat.
• agents.close already gives all of this back: 47 fds, then 59/65/71 with 1/2/3 chats open, then 47 after closing, the same on every cycle.

Fix

The idle sweep keeps an open view's conversation only for the appViewHolds (3) most recently used views. An older view idle for appAgentIdle (10 min) is let go the same way a background chat is: its state stays idle, and its next read or send opens it again through the normal ownership check.

• Running turns, waiting questions, crew members and kept services are still kept, as before.
• The 3 most recent views keep the Merge dev (through #2002) into app/v3: relay, two-computer fixes, quit saving #2009 guarantee, so their relay hold is renewed and not released.
• An explicit close still releases at once.

internal/manual/chat/desktop-sync.md is updated to match.

Measurements

Standalone codeaf app on macOS. Before is the soak binary (app/v3 3a6aa31 + #2466); after is this branch. 10 seeded chats, each opened with agents.open and then left idle, no desk UI:

	before	after
fds at baseline	44	44
fds with 10 chats opened	108	107
fds after 11.5 idle minutes	107	64 (3 views held)
open 10, then close 10	44 → 107 → 44	44 → 107 → 44

Tests

• New TestAppIdleViewsBeyondTheRecentHoldsAreLetGo. It opens 7 views and checks that all are kept within the idle rule. Past the rule, it checks that exactly the 3 most recently used are kept, that further opened-and-idle chats never grow the holds, and that every other open is released once.
• Run 3 times: go test ./cmd/codeaf -run '^(TestAppIdleViewsBeyondTheRecentHoldsAreLetGo|TestAppLetsAnIdleConversationGoAndTheNextSendReopensIt)$' -count=3.
• Run once: the idle-release, relay-fence and lazy-open tests (TestAppIdleRelease.*, TestAppBackgroundE13.*, TestAppLazy.*, TestALiveServiceKeepsItsAgentThroughTheIdleSweep), go vet ./cmd/codeaf, gofmt and go test ./internal/manual/.

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