Unify the preview rail onto one tab list

[OutThisLife]

The right rail had two preview systems living in one tab strip: a list of
file tabs, plus a privileged "live preview" slot with a hardcoded preview
tab id backed by a separate session-keyed registry. Artifacts had recently
arrived as a third list with their own pane and renderers. This collapses
all three into one list, then fixes two bugs that only became visible once
there was a single path to look at.

One list, one way in

Everything the rail can show is now a PreviewTarget in $previewTabs,
and openPreview is the only entry point. $previewTarget is a computed
read of the active tab; the session registry and the effect that reconciled
it against the tabs are gone.

That reconciler is what made open_preview unreliable. The two systems
resolved "which session is this for" under different rules, so a tool
opening a preview in a session whose stored id hadn't landed yet would set
the target and then immediately clear it — the pane flashed and vanished.

Artifacts render in the real preview pane through the shared mode switcher
and source view rather than a parallel pane, with a version stepper in the
header's trailing slot. Their tabs stay memory-only since the registry
rebuilds from the transcript on load; file and URL tabs persist like they
did before.

Composer and transcript state follows its own session

$activeSessionId only ever holds the primary chat's session, but surfaces
that render once per transcript were reading it as if it meant "the session
on screen." A preview produced inside a session tile got recorded under the
main chat's key and showed up in the main chat's composer.

The tool row now records under its own SessionView, along with the other
per-surface readers of that atom: attachment pills and inline preview links
resolve relative paths against their session's cwd, composer voice and
auto-speak read and subscribe to their own transcript, and the thread's
compaction label, prompt-wait gate and turn timer follow the session that
mounted them.

Previews open without the file tree

The preview pane shares a collapsible column with the file tree, and
revealTreePane un-collapses a column through that column's bound store —
which on the right is $fileBrowserOpen, the tree's own Cmd+J toggle. Every
preview open was calling setFileBrowserOpen(true) and bringing the tree
with it. The reveal now un-collapses the column directly and leaves the
toggle alone.

Test plan

• open_preview from the main chat opens the rail and it stays open
• open_preview from a session tab opens the rail (the event used to be
  dropped, since it was gated on the primary chat's session id)
• The preview chip lands in the composer of the tab that produced it
• A preview opens without the file tree appearing; Cmd+J still brings the
  tree in beside it
• Artifact cards open as named tabs, version stepper follows new versions
• File, URL and artifact tabs coexist; re-opening re-fronts instead of
  duplicating
• File and URL tabs survive relaunch; artifact tabs rebuild from the
  transcript
• tsc, eslint, and the desktop suite green

Two related bugs are deliberately left out: native notifications and the
background-delegation notice read the same primary-only atom, so you get a
completion toast for the tile you're looking at. Fixing them means importing
$focusedRuntimeId into those leaf stores, which drags in session-states
and its module-level layout side effects — that hung unrelated suites. It
needs those side effects untangled first.

[github-actions]

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