feat: "panes" close-through; fix: tab closes never counted as user intent (#57)

* feat: "panes" close-through; fix: tab closes never counted as user intent

close_remote_on_local_close grows a middle setting: "panes" closes
panes and tabs through to the remote while a workspace close only
stops mirroring — one wrong prefix+x can cost a pane, never every
agent in a workspace. TOML accepts true (everything, the default),
false (nothing), or "panes"; an unrecognized value warns and lands on
the safe end rather than silently becoming the destructive default.

Separately, close-through never worked for TABS at any setting:
closing a tab emits only tab_closed — no pane_closed for the panes
inside it — and the local watcher never subscribed to it, so a tab
close always degraded to tombstoning. The watcher now records
tab_closed, and the converge claims a missing pane through its tab's
mapped local id as well as its own.

* drop the panes tri-state, keep the tab-close fix

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Co-authored-by: niko <nikokristanto24@gmail.com>