Review findings on #120916:
- MAJOR: `$sidebarSessionOrderIds` is keyed by the LIVE session id — the drag
path persists `reorderableRowIds` (`session.id`) and the sidebar reconcile
effect keeps only ids present in `unpinnedAgentSessions.map(s => s.id)`.
The row slots hand plugins the DURABLE (lineage-root) id, so feeding those
back into `reorder` dropped every compressed session from the order and,
when none survived, flipped `manual` off on the next render. `reorder` now
maps each id to its loaded row's live id through `sessionMatchesStoredId`
(the same matcher pin uses). Test: reorder with `_lineage_root_id != id`
was red on head (`['c','a','b']` written verbatim), green now.
- minor: the Pinned drag path (`setPinnedSessionOrder`) had no verb, so
drag-to-pin-session could not port its pinned reorder. `reorderPinned(ids)`
delegates to it after durable-id resolution; unmentioned pins keep their
slot (test red on head: property did not exist).
- minor: `durableSessionPinId` already resolves a loaded id through lineage;
the unloaded pass-through is now stated in the docs (pass the slot's durable
id for rows that may have scrolled out).
- shape: the ~55 verb lines appended to `sdk/index.ts` move to
`sdk/sessions.ts` (`sessionsHost`); index.ts is import + `sessions:` +
the SESSION_ROW_AREAS export.
- docs: `reorderPinned` in both API blocks; `SESSION_ROW_AREAS` /
`SessionRowSlotContribution` in the exports-at-a-glance table.
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