User terminal tabs and their recent scrollback now survive an app restart
(VS Code parity). Tabs, active selection, cwd, and a serialized scrollback
snapshot are written to localStorage on every change; on launch the tabs
reopen with their history replayed above a fresh shell. Processes are NOT
revived — a new shell starts one line below the restored block.
- Capture: SerializeAddon snapshots the buffer on a 750ms leading-edge
throttle, so a `cmd; quit` lands on disk before teardown; the snapshot is
trimmed of its trailing idle prompt (no "double prompt" on restore) and
capped (200 scrollback lines / 48k chars) to stay under the storage budget.
- Teardown guard: app quit/reload kills the PTYs from the main process,
firing onExit in the renderer, but React skips effect cleanups on teardown
so the per-instance `disposed` flag never flips. A pagehide/beforeunload
flag stops onExit from calling closeTerminal() and wiping the persisted
tabs right before relaunch restores them. A real `exit`/Ctrl-D still closes.
- Agent mirror tabs stay runtime-only — only user tabs persist.
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