`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
