perf(workstation): release idle content hosts when the tab pool empties

[Harry19081]

Summary

Bounds the AppShell's host keep-alive so an idle WorkStation stops holding every previously-visited surface in memory. With no real tabs (Launchpad only), the code / browser / project hosts and the hidden Agent Station simulator all unmount; while real tabs exist, visited hosts stay mounted-but-hidden exactly as before so tab switches remain instant.

Problem

The keep-alive in AppShellContent was monotonic: once a host mounted it never unmounted (visitedModes only ever grew), the code host was mounted from launch even on the empty Launchpad (file-tree autoload, git/diff/diagnostics/output hooks all live), the Browser host was unconditionally pre-mounted one frame after startup, and the Agent Station simulator stayed mounted forever after one visit — even back in My Station with no session. An idle window paid RAM and background-work costs for surfaces showing nothing.

Solution

• New pure policy module AppShell/hostMountPolicy.ts + unit tests covering the mount matrix.
• New derived booleans mainPaneHasRealTabsAtom / mainPaneHasBrowserHostTabsAtom in store/workstation/tabHost.ts (Launchpad start tabs never count as real work; subscribers only re-render when the boolean flips).
• useAppShellDock clears the visited set when the pool empties and drops the deferred Browser pre-mount; the active host still mounts synchronously via the is*Mode branch, so first-tab-open latency is unchanged.
• Browser host mounts on any of: active/visited (with real tabs), background browser-session tabs, live engine sessions, or a pending New-Browser request — the request consumer in BrowserLayout was already remount-safe via the module-scoped consumed-tick atom, and the engine-sessions trigger bridges the gap between request consumption and the sessions ↔ tab-strip sync creating the tab.
• Agent Station simulator: mounted while visible; kept warm while hidden only when a session is attached (workstationActiveSessionIdAtom), so mid-session My Station ↔ Agent Station toggles stay instant but an idle simulator releases its subtree.
• The Launchpad itself now unmounts while real tabs are shown (it was previously kept hidden in the DOM).

Potential risks

• Opening the first real tab from the Launchpad now cold-mounts its host: one file-tree reload IPC and the known one-frame blank in the 40px header strip (same as the current very first launch frame). No grace delay was added for close-last-tab → reopen churn; can be added later if it's noticeable.
• The Browser host's lifecycle has the most triggers; the launchpad New-Browser flow, restored localStorage sessions, and background browser-session tabs are each covered by an explicit mount trigger, but this is the area to watch in manual testing.
• Component-local state inside a host (e.g. editor right-panel width) resets after a full pool-empty cycle; persisted atom state (sidebar widths, layout mode) is unaffected.
• RAM savings are unmeasured in this PR (heap snapshot on the idle Launchpad before/after is the right check); the structural claim — fewer mounted subtrees and no idle file-tree autoload with zero tabs — holds by construction.

Validation / Test plan

• tsc --noEmit clean; eslint clean on touched files.
• vitest run src/modules/WorkStation/AppShell/ src/store/workstation/ — 23 files, 228 tests passed, including the new hostMountPolicy.test.ts matrix (empty-pool release, pending-request mount, session-bridge gap, agent-station session gating).
• Manual checks recommended before merge: New Browser from empty Launchpad; restart with restored browser sessions; close last tab → Launchpad → reopen file tab; Agent Station toggle with and without a live session.

[Harry19081]

orgii://cloud/session/ref?v=1&org=bfa7b134-2486-45fa-81ad-a369441fafb4&owner=e17cf230-b78f-4b1b-8480-6005832983b7&session=claudecodeapp-fc51ef5d-d978-4309-88ae-f69b15f534b4