refactor(model picker): adopt upstream environment favorites, drop the starred group #50

Decided 2026-10-14 (docs/fork/README.md, Decisions): model preferences follow upstream.

Upstream moved favorites and hidden models to environment settings (pingdotgg#16816, f845895271) and added per-project providers (8ac6846a0b). On the bf8ffe81a6 sync these caused 9 of 15 conflicted files (mobile ThreadSettingsSheet, ThreadSettingsRows.shared, thread-settings-sheet-state.test, modelOptions, mobile-preferences, ThreadComposer; server OrchestratorMcpService; web ModelPickerContent; contracts settings.ts).

Work:

• Remove the cross-provider starred group (collectStarredModels, starredHeader/starredModel catalog items) in favor of upstream's favorites tab and @favorites filter.
• Keep commit-on-dismiss (resolveDismissAction, resolveModelPickerCommit) and its guard.
• Move modelCostDisplay out of ClientSettingsSchema/ClientSettingsPatch in packages/contracts/src/settings.ts into a fork-owned client key, so the shared contract stops conflicting. Mobile keeps its device-local preference. State the effect on existing stored settings (old key ignored or migrated).
• Update docs/fork/patches.tsv guards (model-picker-selection, model-cost-display, mobile-model-pricing).

Done: the fork delta in these files shrinks; guards pass; iPad and web picker show prices and favorites from upstream's store.

Supersedes the starred-group half of #9.