fix(ui): let restore paths report no effort without pinning Default

`commitVariantSelectionForModel` turned every `undefined` into an explicit
`Default` (`null`), but it serves two kinds of caller. The picker means "the
user chose Default"; the history and manual-override restores mean "nothing
was found". Restoring a session therefore recorded a choice nobody made, and
`resolveModelVariantSelection` collapsed that `null` back to `undefined`, so
the next restore recorded it again. Because an explicit `Default` outranks the
agent and settings defaults by design, the session latched onto `Default` and
the concrete effort its own history carried could not come back.

Move the decision to the callers: the four picker paths pass `variant ?? null`,
the restore paths pass their result through, and the resolver returns the
selection store's three states instead of two. The follow-up write in the
history restore goes with it — the apply above it already recorded the same
agent and model, and a second write could only disagree.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZuVVgziiLjD81W5vaxdH2

packages/ui/src/stores/DOCUMENTATION.md

@@ -232,6 +232,13 @@ per-session selection store records an explicit `Default` (as `null`) instead of
 clearing the entry — a cleared entry is indistinguishable from never having
 chosen, and the settings default wins again on the next agent or session switch.
 
+Only a place where the user chose may write `null`. Restore paths — message
+history, a preserved manual override — pass their own "found nothing" through
+as `undefined`, because a session whose history carries no effort is not a
+session where `Default` was picked. A restore that manufactures `null` latches
+the session onto `Default`: `null` outranks the agent and settings defaults by
+design, so the concrete effort it displaced can never come back.
+
 Every write of `currentVariant` writes `currentVariantSelection` with it. They
 are one selection; updating only the effective value leaves the picker showing
 one effort while sends carry another.