feat(island): Boite's model picker and effort chip for the assistant

[klNuno]

What changed

Choosing a model in Bots now looks and behaves like Boite's composer model picker.

• src/components/model-picker/: Boite's ModelPicker, ProviderTiles, AccountSeats and EffortSlider, copied from Boite at 4b6335b1 (each file names its source). Same chip, same popover: provider rail with logos, favourites tile, search past 12 models, prefix groups, show-all fold, legacy fold, account seats, refresh, skeletons, keyboard and outside click. Leaf modules that are the same in the vendored submodule (ModelSearch, ProviderLogo, floating, Closing, model-list, model-order, model-picker-keys) come from @boite/ui.
• src/lib/model-picker/store.svelte.ts: a PickerStore giving the picker the part of Boite's Store it reads, on top of CoreLink. Favourites are Boite's own list: read from the core's settings and written with settings.set { modelFavorites }, so a star set in Boite shows in Bots and back (favorites.ts copies the types and the merge rules, which the vendored contracts lack).
• CoreLink gains follow(client) (the island borrows the companion's client), a refresh read and a reading state per provider and account, and saveSettings.
• The assistant's ModelMenu is now the picker plus the effort chip, on the agent's selection. A model of another provider or account moves the agent there with its model's default level, as Boite's agent editor does; the effort chip offers the model's own levels. The popover hangs under the chat's head inside the 940x530 island window, and takes clicks there (data-hit, new optional resized() in the view contract).
• CompanionModelPicker.svelte (not wired yet): the companion's model and effort with an "Automatic" tile first (provider null). Props: store: CoreLink, prefs: CompanionPrefs, onchange?: (next: CompanionPrefs) => void; it writes with writeCompanionPrefs.
• .island-tokens maps the remaining Boite tokens the picker uses (surface-3, active, control glaze, reasoning, shadows) to dark values, and theme-aware logos keep their dark colours on the island.
• Boite's settings pages cannot be opened through ui.reveal, so the install and connect actions are left out.
• Demo: the fake core starts with four starred models; scene=menu|models|search|effort open the picker on the favourites, on Claude, on OpenCode searched for nvidia, or the effort chip.

How it was verified

• bun run check: 0 errors, 0 warnings.
• bun run test: 61 files, 459 tests passed, including new tests for the favourites round trip through the fake core (Bots stars, Boite reads; Boite stars, Bots shows; unstar), the read-once, pending and refresh behaviour, the Automatic choice, the companion picker mounted, and moving the assistant to another provider.
• bun run test:scripts passed, bun run check:manifest: accepted.
• Captures of the island at its real window size with the demo poses, compared with Boite's own picker at 4b6335b1 run on its fake client in the same headless browser.

[klNuno]

Note

Clanker Claude Opus 5.5 is responding on behalf of meetsu

landed in #44 (be3e00f).